Normalize authentication errors from SVN.

When failing to authenticate with a repository, the SVN backends will
return an error of "callback_get_login required." This looks like a bug
in Review Board, but is actually just an error message meaning that the
credentials were bad.

We now normalize these errors, converting them into an
authentication-related error. The SSL error handling code already did
this, but now all call sites use it.

Reviewed at https://reviews.reviewboard.org/r/6275/
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/
Merge branch 'release-2.0.x'
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/
Merge branch 'release-2.0.x'
Fix bug linkification.

The change to the bug tracker URL format strings (using --bug_id-- instead of
%s) was made in most places, but `RB.LinkifyUtils.linkifyBugs` was missed. This
change fixes that.

Testing done:
Put some bug links in text fields (description, comments, review header/footer)
and saw that the links were correct both when initially loading the page and
when making changes.

Reviewed at https://reviews.reviewboard.org/r/6276/