Re-structure profiles sections - closes #214 and closes #248

[benfrancis]

This is a proposed re-structuring of the profiles sections, based on the discussion in #214.

Overview of changes:

1. Renamed "HTTP Profile" to "Common Constraints"
2. Moved Default Language, Links and Security Schemes sections to the top level of this section
3. Moved Information Model, Discovery and Errors sections from HTTP Baseline Profile to Common Constraints
4. Merged the Security section from HTTP Baseline Profile into the Security section of Common Constraints and removed the placeholder Transport Security section in favour of the note referring to the Security Best Practices document
5. Added a placeholder section for Semantic Annotations in Common Constraints
6. Moved Privacy Considerations and Security Considerations into their own top level sections at the end of the document
7. Added assertions to the three profiles to say that they require the constraints in the Common Constraints section to be conformed with
8. Renamed a bunch of HTML element IDs to reflect the new structure
9. Re-worded some assertions to reflect renamed section titles

After these changes, at a high level the specification looks like:

• Common Constraints
  • Information Model
  • Security
  • Discovery
  • Links
  • Errors
  • Semantic Annotations
  • Default Language
• HTTP Baseline Profile
  • Identifier
  • Protocol Binding
• HTTP SSE Profile
  • Identifier
  • Protocol Binding
• HTTP Webhook Profile
  • Introduction
  • Identifier
  • Message Format
  • Protocol Binding
• Privacy Considerations
• Security Considerations

If we land these changes then we'll also need to update the Introduction and Abstract to reflect renamed sections. I didn't do that in this PR because it's already large enough!

I also think we should consider renaming "HTTP Baseline Profile" to "HTTP Basic Profile", since the name "Baseline Profile" may now be confused with "Common Constraints".

My comments in #214 (comment) regarding the Information Model and Message Format sections are left unresolved for now.

Note: This diff is hard to read because it touches so many lines, but it's slightly easier if you just view the second commit (the first one is pulled in from #249 and may be re-based out if that lands first).

Edit: I've renamed HTTP Profile Common Constraints to Common Constraints for simplicity, since all the profiles are currently HTTP profiles. We can re-visit if other profiles are added in the future.

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[mlagally]

Thanks @benfrancis for this contribution.
It significantly improves the structure.

[mlagally]

@benfrancis , can you please resolve the merge conflict.

[benfrancis]

@benfrancis , can you please resolve the merge conflict.

Done.

[mlagally]

Approved in Profile call on Aug 31.