Release Review Board 1.7.28.
Fixed the syntax for a command in the 1.7.28 release notes.

The command was showing as a broken link, since in ReST, surrounding
text with single backticks makes it a link. We now use :command:`...`
instead.
Bump to 1.7.29 dev.
Prevent updating a closed review request through the UI.

A closed review request still had a fully-functioning Update menu,
which allowed new diffs and file attachments to be added without any
visible draft banner. While these would internally be part of a draft
and wouldn't be visible to any other users, it was confusing and
conflicted with the fact that all other editable components were
disabled.

The "X" buttons for deleting file attachments and screenshots was also
being shown, but was non-functional. It too has been removed.

Reviewed at https://reviews.reviewboard.org/r/6268/
Fix usage of timedelta.total_seconds() in condensediffs.

The `total_seconds()` method doesn't exist in Python 2.6, so we can't use it
yet. This change replaces it with the equivalent implementation (and moves us
over to using true division throughout this file as a side effect).

Testing done:
Condensed some diffs.

Reviewed at https://reviews.reviewboard.org/r/6304/