Unfriend

* Show trigger in friend detail view
* Ask for confirmation
* Show loading indication while request is processing
* Remove entry from `friends` table

Note: We could technically remove photos where only the two friends involved in the unfriending action are shown. But we shouldn't remove photos where other friends are tagged as well. I'd keep existing photos in the friends feed, since it's too hard to explain otherwise:

No files or tests are named. Start by locating the friend detail view and the request path in the React Native client, then trace how the friends table is updated. Done means the detail view exposes an unfriend trigger, confirms the action, shows loading while it processes, and removes the friend entry without removing shared photos.

Written by the indexing model from the issue text.