Use the User model for foreign keys.

Ghost User
2020-01-29T21:52:44+01:00

Closes no issue.

### Short description

Previously foreign keys pointed at Member objects. This created some issues as `autocomplete_fields` (in Django 2.0) and `raw_id_fields` require an `Admin` for the models that a foreign key points to. As `Member` has no `Admin` this meant that we could not use `raw_id_fields` and `autocomplete_fields` for `Member`.

This MR fixes this issue by replacing all foreign keys to `Member` with foreign keys to `User`.

In particular this MR does the following things:
1. The `events`, `pizzas`, `activemembers`, and `mailinglist` apps now use the `User` model for foreign keys.
2. The `Member` model has been renamed to `Profile` to reflect that it is (mostly) used for storage of extra information.
3. A proxy to `User` called `Member` has been added. This allows us to override `Manager`s and `__str__` on `User`. Most methods that did not make sense in `Profile` have been moved to the new `Member`.