Add an is_enabled_for mechanism for NavigationBarHook.

This adds a way for NavigationBarHooks to be enabled/disabled per-user. This is
done by adding a new, optional is_enabled_for flag to the constructor, which is
a callable that returns either True or False.

Testing done:
- Ran unit tests.
- Used this in an extension.

Reviewed at https://reviews.reviewboard.org/r/6390/
Changed HostingServiceHook docs versionadded entry to version 2.0.12.

Changed versionadded in docs for the new HostingServiceHook to prepare for its
release in version 2.0.12.

Testing Done:
Built docs.

Reviewed at https://reviews.reviewboard.org/r/6687/
Add the missing doc files for the draft original/patched file resources.

These were missing from the tree, but were otherwise referenced in the
documentation. This simply adds the small files pointing to the
WebAPIResource instances.
Merge branch 'release-2.0.x'
Add draft release notes for Review Board 1.7.29

As can be expected from a point release at this stage, 1.7.29 is a pretty tiny
release with just a couple bug-fixes. Here are the release notes.

Reviewed at https://reviews.reviewboard.org/r/6449/
Merge branch 'release-1.7.x' into release-2.0.x
Merge branch 'release-2.0.x'