Add 'preferred currency' info to user data

[djavorek]

What needs to be done

• Add a field for preferred currency info into our user table.
• Modify migrations to provide base data
• Modify registration process to either default to a curreny (probably EUR) or ask it from the user
• Create an endpoint to be able to modify it
• (If profile page exists by that time) It would be good to add it there as well, with the option of changing it

(Needs to be defined)

• How to use this info to avoid curreny conversion at the most places

Why

• Subscription prices are one of our main topic. Showing them in a foreign curreny is really bad UX