Numerous improvements to security considerations

[handrews]

This is probably about as much as we'll do for draft-07. We'll keep working on the security sections as the drafts progress, but this is at least as good as it has been for the last few drafts.

Addresses #456 plus various things not filed as issues.

Commit log:

|     Security considerations for "content*"
|     
|     a.k.a. don't take executable content types from strangers
|  
* commit 58c6b96cb13d5a593209154cc854067c3bf0e954
| Author: Henry Andrews <[email protected]>
| Date:   Tue Nov 14 12:44:59 2017 -0800
| 
|     Add a bit more about target attribute authority
|     
|     While "targetSchema" is technically never required to process
|     a response, as the response should indicate its own schema,
|     other target attributes in the LDO, such as the submission schema,
|     cannot be conveyed in any way other than through the LDO.
|     
|     It would seem like we need a provision for determining when the
|     LDO can be constructed, but it's not entirely clear how that
|     should work.  Put in a bit about it and a CREF to hopefully
|     attract the attention of someone who can improve the section.
|  
* commit b49541da81be4302f20d3f824ce50e8f6787c225
| Author: Henry Andrews <[email protected]>
| Date:   Mon Nov 13 22:06:33 2017 -0800
| 
|     Move "self" link security bit to last subsection
|     
|     The other parts flow much more naturally from the mention of
|     RFC 8288 in the introductory part of the security section.
|  
* commit d7254824f8188a7e8ad8e441ead4f4eeb641d597
| Author: Henry Andrews <[email protected]>
| Date:   Mon Nov 13 22:03:19 2017 -0800
| 
|     Reference RFC 8288 in Security Considerations
|     
|     8288 is the update of 5988 Web Linking.  As a link serialization
|     format, Hyper-Schema shares those vulnerabilities.
|  
* commit 11de781a2b863e36463b242c36e4f526db819a2f
| Author: Henry Andrews <[email protected]>
| Date:   Mon Nov 13 16:26:23 2017 -0800
| 
|     Clean up target attribute security considerations
|     
|     This was an awkward copy-paste.  Fix it up and remove the CREF
|     reminding me to do so.
|  
* commit 379285ab5e3ef1c3a8f0b0033b8759c57740ea8b
| Author: Henry Andrews <[email protected]>
| Date:   Mon Nov 13 16:08:47 2017 -0800
| 
|     Add CREF for "self" link sub-path
|     
|     Since I have no idea what it is trying to do.

See also #485 about possibly removing the "self" link bit.

[handrews]

@json-schema-org/spec-team anyone?

[handrews]

Trying to get this published on Monday or at least sometime next week...

[handrews]

Update just now is a rebase to handle conflicts with the title-casing change.