Unify migration descriptions

[delvh]

• Gitea version (or commit ref): 1.16.0+dev-138-gdb1e3d02a

Description

Currently, there does not seem to exist a standard for the migration descriptions.
I, and most likely everyone else as well would prefer it, if these descriptions would be unified.
What do I mean by that?

1. I would replace Migrating data … with its more precise version Migrate data … as currently no action is already being performed
2. Some URLs are started with a capital letter, and some are not (example: Github.com, notabug.org). Either all URLs should start with their first letter capitalized, or none at all. In the first case: How do you handle urls with subdomains (especially code.onedev.io)? Do you capitalize all domain starting letters, or only the first?
3. Unify the overall descriptions: Currently, the unique descriptions each differ slightly enough to be noticeable. I think it would be better to use a unified approach for everything such as Migrate data from <Provider.URL> or other <Provider> instances.
4. Cut the or Mirroring from Migrating or Mirroring git data from Git services in migrate.git.description. All other migration descriptions also enable this and don't publicly announce it.

Screenshots

Current appearance (in 1.15, not current dev 1.16)

Expected result afterwards

[zeripath]

I think it's probably best just to send a PR updating locale-en_US (with screenshots - although I know that's going to be annoying to do.)