Manage restart message on reverting to initial username

Take into consideration also the following usecases:
 - user also changes the color
 - user cancels the modifications and reverts to the initial username
 - user makes a change, reverts, clicks accept, reenters the About Me settings

I have made some additional changes by making `self.restart_alerts` a set. As a list it accumulates elements with same values and a simple remove doesn't make a difference. 

Now both testcases work as expected.