Fix context leaks when using Context.update().

Context.update() implies a push(), which some of our code wasn't
accounting for, resulting in leaked state This mainly impacted
extensions, though datagrids were also doing this (not resulting in
leaks, but rather slightly slowing down rendering).

We now construct a Context as late as possible, instead of creating one
early and operating on it, and pop() when necessary.

Reviewed at https://reviews.reviewboard.org/r/5763/
Add the ability to remove source media files when building extensions.

When packaging extensions, it's now possible to strip away all source
media files (non-minified JavaScript/CSS/LessCSS files) from the
resulting package. These are generally not needed, and doing so can
reduce the size of the package.

There's a 'remove-source-files' option that can be set in setup.cfg that
will cause all JavaScript/CSS/LessCSS source files to be removed from
the build. This can be set by adding the following to setup.cfg:

    [build_static_files]
    remove_source_files = 1

Reviewed at https://reviews.reviewboard.org/r/5775/
Release Djblets 0.8 RC 2.
Bump to 0.8 RC 3 dev.
Fix populating extension settings forms from defaults.

The move to SiteSettingsForm broke extension configuration pages
slightly. They accessed the settings object using settings.get(key), and
not through settings[key], which prevented any defaults from being
fetched.

There's now a get() function that works just like attribute access,
returning defaults, but with the semantics of the standard dict.get()
call. This complements set().

Reviewed at https://reviews.reviewboard.org/r/5805/
Bump required version of django to 1.7.13

Picking up the latest security release.
Merge branch 'release-0.7.x'
Bump django dependency to 1.6.5

Picking up the latest security release for Django.