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/
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.
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