Make 'rbt --help' be a little more sensical.

The help output for rbt would say "the most commonly-used commands are:" and
then proceed to spit out an unordered list of every command, no matter how
commonly used.

I've replaced the "most common" commands list with a curated list of the things
that I think are actually most common, in the order that people probably care
about them. We then print out the rest of the commands in alphabetical order.

Testing done: Ran rbt --help

Reviewed at https://reviews.reviewboard.org/r/4987/
Enhance rbt post to automatically determine the existing review request

``rbt -u`` automatically determines the existing review request to update. It
suggests a pending review request by the current user in the current
repository, based on the request summary and description.

If the summary and description exactly match those of an existing request, the
request is immediately updated. Otherwise, the user is prompted to confirm a
suggestion from a ranked list of potential matches.

Testing Done:
- Tested with Git, Mercurial, and Bazaar using, with and without 'revision
  range'.
- Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/4699/
Do some trivial rearranging to the rbt docs.

This change moves 'rbt' ahead of 'post-review', changes the "beta" notice on
the rbt page, and adds a deprecation warning on the 'post-review' page. Really
I just wanted the fish.

Testing done: Built HTML and looked at it.

Reviewed at https://reviews.reviewboard.org/r/4994/
Flatten rbtools docs a bit.

The structure of the rbtools docs made it really unclear how to actually see
information about using the tools, since the table of contents just had stuff
about installation.

I've pulled the rbt/index and api/index files into index itself, under a "Using
RBTools" section. At least this way, the top-level TOC has something other than
"Installation", and there are a bunch of links visible as soon as you see the
page to the specific commands, instead of having to drill down through several
pages.

While I was doing this, I pulled in the info from "Automating post-review" in
the old tools section of the Review Board user manual, which seemed to be the
only thing present in that that wasn't here.

Testing done: Built html docs and looked through them.

Reviewed at https://reviews.reviewboard.org/r/4997/