Enable public and private user profiles, tweaks to user registration

# What's new
* `is_private` field on User model (first steps on making #262 implemented).
* Using [pundit](https://github.com/elabs/pundit) for permission checking on viewing users.
* Added a fallback to english for translations. This doesn't seem to work all the time yet.
* Tests for new permissions things.

# ~~In progress~~
* [x] test for password change when deleting user account
* [x] test that users are shown and not shown, not just based on the policy.
* [x] Will have to investigate whether the automatically generate ApplicationPolicy is necessary.
* [x] Figure out how to make pundit work alongside mutations

# What's next
* Figuring out gardens.