App structure repeater

This adds:

1. `AppStructureRepeater` with a little test that I gleaned by reading the `receiverwrapper` tests. It only forwards the app id, as a notification that the API should be polled, because the app structure is not in one piece anywhere in HQ except where the API assembles it.
2. Support in the views & templates for the `domain` app for editing this endpoint, adding a test suite to the app to test this (manually QA'd first, then wrote a test for it)