* reject usernames made only of dots
A username of "." or ".." passed validation, but browsers resolve those as path segments, so requests to /users/{username} never reached the API and the user could not be deleted or edited from the UI. User creation now rejects dot-only names in both the API and the create user dialog.
* show a specific error for dot-only usernames
Entering "." or ".." in the create user dialog showed the message listing periods as allowed characters, which didn't explain the rejection. The dialog now checks for dot-only names separately and shows its own message.
