DOCS: Add version chooser to theme

This requires tweaking the theme to add some elements, plus a version of
our previous javascript to populate it based on json in the root of the
docs repo.

Author: dopplershift

docs/_static/doc_shared.js

@@ -0,0 +1,43 @@
+const proj = "MetPy";
+
+$(document).ready(function() {
+    cur_ver = DOCUMENTATION_OPTIONS.VERSION;
+    end = cur_ver.lastIndexOf('.');
+    if (end > -1) {
+        cur_ver = 'v' + cur_ver.substring(0, end);
+    }
+    console.log('cur_ver: ' + cur_ver);
+
+    $.getJSON('/' + proj + '/versions.json', function(data) {
+        if (cur_ver !== data.latest) {
+            let msg;
+            if (cur_ver.includes('dev') || data.prereleases.indexOf(cur_ver) > -1) {
+                msg = 'development / pre-release';
+            } else {
+                msg = 'previous';
+            }
+            content = $('<div class="version-alert">This documentation page is for a ' + msg +
+                        ' version. For the latest release version, go to <a href="https://unidata.github.io/MetPy/latest/">https://unidata.github.io/MetPy/latest/</a>');
+            $('#banner').append(content);
+        }
+
+        $.each(data.versions, function() {
+            if (this !== 'latest') {
+                const url = DOCUMENTATION_OPTIONS.URL_ROOT + '../' + this;
+                const name = this.startsWith('v') ? this.substring(1) : this;
+                $('#version-menu').append('<a class="dropdown-item" href="' + url + '">' + name + '</a>');
+            }
+        });
+    });
+});
+
+// Borrowed from Bokeh docs to look for an alert.html at the base of the
+// docs repo and add that to the banner if present.
+$(document).ready(function () {
+    $.get('/' + proj + '/alert.html', function (data) {
+       if (data.length > 0) {
+         content = $('<div class="news-alert">' + data + '</div>')
+         $('#banner').prepend(content);
+       }
+    })
+ })

docs/_static/pop_ver.js

@@ -0,0 +1,62 @@
+<div class="container-xl">
+
+    <a class="navbar-brand" href="{{ pathto(master_doc) }}">
+    {% if logo %}
+      <img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="logo" />
+    {% else %}
+      <p class="title">{{ project }}</p>
+    {% endif %}
+    </a>
+    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbar-menu" aria-controls="navbar-menu" aria-expanded="false" aria-label="Toggle navigation">
+        <span class="navbar-toggler-icon"></span>
+    </button>
+
+{# Removed col-lg-10 to match align:left from next version #}
+    <div id="navbar-menu" class="collapse navbar-collapse">
+      <ul id="navbar-main-elements" class="navbar-nav mr-auto">
+
+{# Begin adding version chooser #}
+        <li class="nav-item dropdown">
+          <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
+            Version {{ version }}
+          </a>
+          <div class="dropdown-menu" aria-labelledby="navbarDropdown" id="version-menu"></div>
+        </li>
+{# End #}
+
+        {% set nav = get_nav_object(maxdepth=1, collapse=True) %}
+        {% for main_nav_item in nav %}
+        <li class="nav-item {% if main_nav_item.active%}active{% endif %}">
+            <a class="nav-link" href="{{ main_nav_item.url }}">{{ main_nav_item.title }}</a>
+        </li>
+        {% endfor %}
+        {% for external_link in theme_external_links %}
+        <li class="nav-item">
+            <a class="nav-link nav-external" href="{{ external_link.url }}">{{ external_link.name }}<i class="fas fa-external-link-alt"></i></a>
+        </li>
+        {% endfor %}
+      </ul>
+
+
+      {% if theme_search_bar_position == 'navbar' %}
+        {%- include "search-field.html" %}
+      {% endif %}
+
+      <ul class="navbar-nav">
+        {% if theme_github_url | length > 2 %}
+          <li class="nav-item">
+            <a class="nav-link" href="{{ theme_github_url }}" target="_blank" rel="noopener">
+              <span><i class="fab fa-github-square"></i></span>
+            </a>
+          </li>
+        {% endif %}
+        {% if theme_twitter_url | length > 2 %}
+          <li class="nav-item">
+            <a class="nav-link" href="{{ theme_twitter_url }}" target="_blank" rel="noopener">
+              <span><i class="fab fa-twitter-square"></i></span>
+            </a>
+          </li>
+        {% endif %}
+      </ul>
+    </div>
+</div>

docs/_templates/docs-navbar.html

@@ -0,0 +1,6 @@
+{% extends "!layout.html" %}
+{% block docs_navbar %}
+    {{ super() }}
+    {# Added to support a banner with an alert #}
+    <div class="container-fluid" id="banner"></div>
+{% endblock %}

docs/_templates/layout.html

@@ -220,7 +220,7 @@
 html_static_path = ['_static']
 
 html_css_files = ['theme-unidata.css']
-html_js_files = ['pop_ver.js']
+html_js_files = ['doc_shared.js']
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied