Add a 'I forgot my username' link to the login screen which allows a user to retrieve (via email) the user name(s) associated with that email address.

I have written a patch that provides that feature and I will upload it to git.

The interface and internal mechanisms are similar to those for 'I forgot my password'.

I believe that this patch solves (using a different mechanism) the problem raised in https://tracker.phpbb.com/browse/PHPBB3-10432