Add top level infrastructure for new extensions work
Add an extension to customize email through wagtail. This was originally implemented in !53 (merged), but this moves the functionality into an extension.
One thing that does that's different from !60 (merged) is that it adds a new django signal to the hypha codebase, which it then uses. So between !60 (merged) and this MR, a whole picture of how we will add extensions can be gotten.