Add username

[pcrespov]

In view of the comments in PR #1572, the design of users-api and the constraints imposed on the database needs a deep review to avoid inconsistencies of dangling projects/groups

• move from email to username as unique identifier of a user (a user can still login with one/multiple emails in the future)
• can be used to reference any user in chats/notes by using @username : api to search user

Review as well:

• see _get_user_name during registration
• split name into first_name, surname and leave the latter for a potential username. Create migration
• functions to format human-readable names/surnames to address users in communications

[sanderegg]

@pcrespov still a thing?