support different email for signin

Now barkeep thinks my gmail address for signin is the same to the email address in my commits. This is not the case. 

I've introduced a new login field for signin, and made current email address editable.
Just the signin process changed slightly, everything works as before. Except you can configure your email address. 
As before, all outgoing mail sent to this address, and the commits have to contain this address too.
A migration also included to fill the new login field to the current email, so existing users works after this change.

In general, this is another apporach to issue #426