Refactor Core things, including Ecto handling, ActivityPub & Transmogrifier modules

• Data doesn't need anymore to be converted to ActivityStream format to be saved (this was taken from Pleroma and not at all a good idea here)
• Everything saved when creating an event is inserted into PostgreSQL in a single transaction
• Also closes #256 (closed)

Warning

To migrate existing data, the following commands have been added.

• mix mobilizon.setup_search
• mix mobilizon.move_participant_stats