Fix saving extension settings.

A recent change added a 'request' parameter to SiteSettingsForm, and
changed the calls in siteconfig to pass the 'data' and 'files'
values as keyword arguments.

This broke the form, because extensions use the configure_extension
view, which passes data as positional arguments.

While that view could also just be updated for this, the convention is
to pass 'data' and 'files' as positional arguments, and most callers do
so. To provide the best compatibility, we now do this as well, and leave
optional keyword arguments (request) as keyword arguments accessed
through `kwargs`.

Reviewed at https://reviews.reviewboard.org/r/5967/
Fix the extension 'enabled' state in the API after enabling/disabling.

A recent regression caused ExtensionResource's 'enabled' state to be
incorrect after enabling/disabling an extension. The value would be that
of the old state. This led to an unexpected combination of states in the
extension list, which caused the "Enable", "Disable", etc. actions to
disappear.

The reason this happened is that get_object() now caches results, and
ExtensionResource was attempting to use it in order to reload the
RegisteredExtension instance. We now reload it like all other resources
do, which is to just re-fetch from the database using the existing
instance's ID.

Reviewed at https://reviews.reviewboard.org/r/5969/
Merge branch 'release-0.8.x'