Move the Djblets release notes into ReST files.

This removes the old NEWS file and splits up all the release notes into
ReST files. These can later be shown directly on the Review Board
website, instead of having to go through the NEWS files on
downloads.reviewboard.org. This will also make it easier for us to link
to Djblets release notes in other docs.

Each of these files were copy/pasted from their NEWS sections, with some
macros and regexes used to transform them into the proper format. I also
went through some of them and fixed up styling, and added appropriate
ReST markup where needed.

Reviewed at https://reviews.reviewboard.org/r/6067/