Add Validation to Forgot Username

Added a ```required``` HTML5 attribute to prevent submitting an empty username
and we throw an appropriate error if a blank username is sent to the
server. Fixes #1374!
Merge branch 'topic/forget-username-fix' of git://github.com/emstellato/OTM2 into emstellato-topic/forget-username-fix

Conflicts:
	opentreemap/treemap/views/user.py
Merge branch 'emstellato-topic/forget-username-fix'