[WIP][PoC] Customer details split

A customer could...
- registed on the site,
- purchase with us and register during the process,
- purchase with us and NOT register during the process,
- contact us via an online form,
- enter a competition,
- sign up for our newsletter,
- be an admin user.

TODO:
- [ ] gather feedback,
- [ ] add migration files,
- [ ] add backend management for Customers,
- [ ] adjust missed parts of internal code to match recent changes
Parameter for class ?