For some reason, member invite tokens can be cleared without saving an
associated user. These entries cause 500 errors to show up in the member
view, and they can't be fixed easily without manually deleting entries
in the database.

This commit does two things:

1. Remove these orphaned entries from the database.

2. Gracefully handle them in the code: log an exception with details and
   provide a message in the remove button that allows project/group admins
   to delete these entries.

On GitLab.com, the migration removes 19 rows with the latest orphaned
entry created on 2020-04-16. That suggests the underlying bug is still
there but does not happen very often.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/23198