Remove an unnecessary external image from the review request email.

We use an image for the background of top of the review request table, but it
doesn't add much, and external images in email are annoying.

Testing done: Ran tests. Viewed emails.

Fixes bug 3080.

Reviewed at https://reviews.reviewboard.org/r/5182/
Merge branch 'release-1.7.x'
Improve our handling of file encodings with diffs.

The way that we dealt with diff and file contents was kind of broken with
regards to special encodings. This was made worse by the unicode transition,
and in particular, I noticed that the diff validation resource would raise an
exception if the diff had unicode characters in it. Fixing that inspired me to
fix all our issues.

We now decode strings to unicode before manipulating them, and then encode them
back to bytes afterward using the encoding that we used to decode. This means
that we're no longer splitting lines or doing regex replacements on bytes, only
on unicode objects.

Testing done:
* Ran unit tests
* Uploaded a diff containing unicode text via the "New Review Request" form.

Reviewed at https://reviews.reviewboard.org/r/5161/
Fix the dashboard's multi-selection with reloading.

There were a couple bugs causing multi-selection to break when
reloading.

First, the timer pausing logic wasn't working, due to a typo in a
variable name. This also resulted in toggling a checkbox causing
multiple timers to start and stack.

Second, after a reload, the events for the actions in the sidebar were
no longer hooked up. This is fixed by a simple delegateEvents() call.