Smarter app hook ui (related to issue #2612)

A slightly smarter UI for the AppHook portion of the Advanced Settings for a Page.

With this PR, the "Application instance name" fieldrow does not appear unless the selected "Application URL" requires one.

If the user selects an Application URL that requires an instance name, it will be prepopulated with the "default" instance for her.

Tests pass.

This should be ready to merge, but will let someone else review first.