Fix support redirect for local sites.

The "contact support" link wasn't expecting to get a 'local_site_name'
argument, and so it caused a 500 error. This change just makes it accept all
kwargs and ignore them.

Reviewed at https://reviews.reviewboard.org/r/6279/
Fix javascript errors when changing diff revisions too fast.

When the diff revision sliders are moved before all the files on a page finish
loading, it can cause a javascript error. The cause here is that the
DiffReviewable has an outstanding fetch, which attempts to render the file once
it finishes. If the element disappears, we try to dereference things which are
now undefined. I've made it test to see whether the target element is present
before creating the DiffReviewableView, which avoids all of the problems.

Testing done:
Flipped around to a bunch of different diff revisions, changing things before
all files loaded. After this change, everything works swimmingly.

Fixes bug 3514.

Reviewed at https://reviews.reviewboard.org/r/6280/
Fix the display of dashboard-like pages.

The dashboard-like pages (users page, All Review Requests, etc.) broke
with my recent dashboard display fix. They were appearing hidden. We now
only hide by default on the actual dashboard page.

Reviewed at https://reviews.reviewboard.org/r/6283/
Provide additional information for exception traceback e-mails.

This adds new middleware to provide information on the username and
e-mail address of the user who triggered the exception, if any. It also
logs the Local Site name, if one is set.

Reviewed at https://reviews.reviewboard.org/r/6278/