If a user has an existing account with phone number or email as
account ID we show a popup at startup where we require that he sets the
user name. This popup has no close button so he is forced to enter a
value. If there are multiple account multiple popups will be shown.

To not break signed accounts we keep accountId as internal id used for signing.
Old accounts get a popup to add the new required field userName but accountId is
left unchanged. Newly created accounts fill accountId with the value of userName.
In the UI we only use userName.

Input validation does only check for length (5-100 chars). Not sure what
are the requirements at Revolut. Can be changes easily if anyone gets
the specs.

Prefer to keep old functionality here...