Disallow creating review requests against other users' changesets

Review Request #35 — Created May 28, 2007 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
At the moment, it's possible to create a review request against
any changeset (with the perforce backend).  If one person
creates a review against another person's changeset number, that
will then prevent the owning user from creating a request.

Since perforce changelists include usernames, it's not hard to
parse this out before we create anything in the database and
check it against the logged-in user.

Part of this change also fixes a bug in the "New Review Request"
form where the selected repository was not being marked as such
if there was an error.
Tried creating a review request against public.perforce.com with
a random change number and my local user ("david").  Got expected
error message.