Add location & registration date support to the user

Two new fields should be added to User entity: user location (just a simple string, limited to 30 chars) and registration date. Registration date should be calculated automatically, while location should be eligible for edition via user profile page.

Modification scope may be defined as follows:
1. Alter database table using new flyway migration
2. Complement User entity class and related DTOs/Containers
3. Modify service layer to autoassign redistration date
4. Update user profile field to contain new input field