An F3 user is a new type to represent a user created because it is
referenced when importing an F3 archive. It is associated with the
forge from which the F3 archive originates. For instance if importing
an archive from GitLab.com, a user will be created as if it was
authenticated using GitLab.com as an OAuth2 authentication source.
When an actual user authenticates to Forejo from the same
authentication source, the F3 user will be promoted to be a regular
user that owns all issues, pull requests, comments etc. created using
the F3 user.
