Fix and improve move detection.

Move detection had some issues that were a result of a stale variable
being accessed. When setting metadata, a previous variable was used,
instead of the instance we were currently operating on. This led to a
bunch of move information getting tacked onto the wrong line and erasing
things. That's been fixed, so move information can be better trusted.

Move detection was also picking up lots of little lines we really didn't
care about. It now has a threshold. It must be at least 2 lines of code,
or 1 line at least 20 characters in length. We may decide to tweak these
later on.

There was also an off-by-one with move ranges. If a range ended with a
line that wasn't considered interesting enough (< 4 characters), but the
range otherwise passed validation, that uninteresting line would be
included. This isn't always bad (might catch an ending brace, for
example), but it's not intended, and shouldn't be provided unless we
design for it.

I also made the move flags more consistent between the left-hand-side
and right-hand-side of the diff viewer. Both the "moved from" and "moved
to" flags are now anchored on the left of their side, so it's easier to
see what lines they span. Basically, I'm applying the new changes
recently made for the right-hand side to the left.
Better optimize for empty Markdown text fields.

Empty Markdown text fields still had a loading indicator (which was
partly cut off) and ran through all the linkifying and conversion steps.
We no longer show the loading indicators in this case, or do any
processing.
Stop trying to link the old Djblets directory during install.

rb-site install and upgrade were still trying to link the old Djblets
path, which we don't need anymore. When it failed, it would log an error
to the console, though it was harmless. We can safely remove all that.
Prevent update bubbles when resolving/reopening issues.

When resolving or reopening an issue, the code was marking the review
request as updated, using the timestamp coming from the response.
However, this timestamp wasn't actually factored into the review
request's last activity timestamp, so it ended up popping up an update
bubble showing the actual previous update.

That code is just gone now. There's no longer any update bubbles when
resolving issues.
Fix clicking on move flags in the diff viewer.

Move flags haven't been clickable in a while. They were still calling
the old RB.scrollAnchor function. Now, clicking emits a moveFlagClicked
event, which the page listens to and scrolls to the matching anchor for
the other end of the move range.
Fix places where we were using relative imports.

Relative imports won't be allowed in Python 3, and they sometimes cause very
mysterious bugs (like decorators being applied twice).

Reviewed at https://reviews.reviewboard.org/r/4929/