Allow callers to override the inlineEditor multi-line text field.

inlineEditor now works with custom text fields for multi-line text. A
caller can provide options for overriding the creation of a text field,
setting text, and getting text. Through this, it can place its own
widget (or even just a more complex object still wrapping a textarea).
Bump AJAX_SERIAL when bumping the Extension sync generation.

Extensions often affect many aspects of page rendering. Pages that make
use of AJAX_SERIAL in their cache keys expect that a change to a
template will cause a change to the cache key. It therefore makes sense
to bump AJAX_SERIAL when we bump the extension sync generation (which
happens when enabling/disabling an extension or changing extension
configuration).

With this change, any pages that properly use AJAX_SERIAL will
invalidate their cache any time an extension makes a change that may,
for example, toggle a feature on or off.
Merge branch 'release-0.7.x'
Release Djblets 0.7.22.
Merge branch 'release-0.7.x'
Update for new exception syntax.

Python 2.6+ introduced a new syntax for catching exceptions with the exception
value. Instead of `except Exception, e`, it's now `except Exception as e`,
which is really much nicer. Since we're 2.6+ only now, we can convert to this.

Testing done: Ran unit tests.
Reviewed at https://reviews.reviewboard.org/r/4875/