Issue 3189: Add a link to the interdiff from the review request draft banner

Adds a link to the interdiff from the review request draft banner to make it
easier to see the change the draft contains.

Testing Done:
* Tested the change using a development review board with different drafts
  (with and without diffsets).
* Unit tests pass.

Bugs closed: 3189

Reviewed at https://reviews.reviewboard.org/r/5384/
Run unit tests from executable Python modules

When developing on a Linux VM inside a Windows host, all Python modules have
the executable flag on. Nose has work around for this when run on Windows
(https://github.com/nose-devs/nose/blob/master/nose/config.py#L26). In this
case, the Python intrepeter run inside the VM causing the work-around to fail.
This review request adds check for executable file permission and forces Nose
to run tests.

This is similar to Djblets review request 5325
(https://reviews.reviewboard.org/r/5325/).

Testing Done:
Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/5383/
Fix highlighting indentation on lines starting with HTML.

Some Pygments highlighters mark up each line with a span tag, which the
indentation highlighter would nibble on as it tried to highlight the
indentation.

It's now smarter, and understands that it may have to share with other
span tags living on that line. If it encounters a span tag as the first
character, it will start looking for the spaces inside, nesting further
if needed.

If it can find all the whitespace it wants to show indentation for, then
it will do so. If it can't (for example, if certain sequences of spaces
were wrapped in a span, but not all), then it will bail and just not
show anything, to be safe.