Add draft 2.0 release notes.

This has everything from the 2.0 beta 1, beta 2, beta 3, RC 1, and RC 2
releases, plus the changes that have gone in since RC 2, minus the changes and
bugfixes that have been backported to 1.7.x, and minus any fixes from later
prereleases for features introduced in earlier prereleases.

Testing done:
* Built HTML and read through it.
* Did some fancy-pants grepping to check for backported bugs.

Reviewed at https://reviews.reviewboard.org/r/5717/
Sandboxing NavigationBarHook get_entries

Added unit tests to test sandboxing

Testing Done:
* Unit testing, added to test suites in reviewboard/extensions/tests.py
* Unit testing with and without the sandboxing

Reviewed at https://reviews.reviewboard.org/r/5708/
Require the latest Django 1.4.11 security release.

This fixes a number of security issues. Most importantly to us, a bug in
reverse() that could lead to importing unexpected modules, if the
attacker knows which modules are on the server.

See https://www.djangoproject.com/weblog/2014/apr/21/security/
Release Review Board 1.7.25.
Merge branch 'release-1.7.x' into release-2.0.x
Merge branch 'release-2.0.x'