As a user, I want to be able to create an account in Patat

# Description
As is, it is only possible to register as a restaurant (that is, create a user  and corresponding location entries in the database). This feature introduces a new type of user — a customer — a hungry person who intends to use Patat to find a place to eat.

# Checklist
- [x] Add a new table for Customers
- [x] Modify the values in the Groups table to differentiate between the users of Patat
- [x] Add a form for Customer sign up
- [x] Add a model for Customers
- [x] Add views for Customer signup and editing
- [x] Modify the main view to add navigation items for registration/profile editing
- [x] Add UserToGroups model for writing a newly registered user to an appropriate group
- [x] Add a check for username uniqueness

# Questions
Navigation items: positioning, naming etc are subject to debate.
Run Customers and Groups scripts to update your database.