Hi Paul:

Please see: https://wiki.roundup-tracker.org/LoginWithEmail.
This implements using the primary email address as a login name.

My earlier comment about using interfaces.py isn't needed. A simple
login action replacement using a file in the extensions directory does 
fine.

Drop the file into the extensions directory and restart the tracker
and you get the functionality.

I chose to not complicate the core code with this. The extension is 
17 lines of code (not including comments/debug) to do the job.

If somebody wants to allow login by alternate email address, they
can use this as a base to work from.

Note this only works for the web UI. API logins via xmlrpc, rest
etc. still require the username.

-- rouilj