Add improved Local Site limiting to IntegrationConfigForm.

Review Request #1577 — Created March 4, 2019 and submitted — Latest diff uploaded

Review Board
`IntegrationConfigForm` now makes use of `LocalSiteAwareModelFormMixin`
in order to gain more comprehensive support for Local Site limiting.
Now, any subclass of this form will automatically have any of its model
relationship or condition fields bound and validated to a Local Site.

This replaces the very limited support the form natively had before.

Testing Done:
Unit tests pass.

Used this with Local Site limiting and verified that everything was
properly filtered and validated against the Local Site.

Reviewed at