Strip the domain name from users created via mail handler

It would be nice if we could set that every new user created via mail handler will have the username stripped starting from '@'.

Nowadays, whenever a user is created through a mail handler, the username of the user become automatically his email address.

For instance, if I have my mail handler set to create new users upon incoming emails and my email is user@atlassian.com, my username in JIRA will be 'user@atlassian.com'. With this improvement, my username will become 'user'.