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/