Assign username on new login

[ddanielgal]

No description provided.

[ferencmarkizay]

• Backend: Add display name to profile and account endpoints, only return full name on /accounts/me (later: add a settings that makes the full name public on the user's profile)
• Backend: Populate script: display name <- full name
• Frontend: If display name is not set, redirect to a page where the user can set their display name (default value: full name?)

[ddanielgal]

We will not use full name at all.
Display name should replace full name.

[ddanielgal]

• Backend: switch full name to display name
• Frontend: If display name is not set, redirect to a page where the user can set their display name. No default.