Updaet user manager

Introduce app-callout styling borrowed from the twbs docs and make
the invitation views actually return useful information
Introduce custom mailer and invitation handler

First draft of a generalized standalone mailer to invite and
notify new users after their account has been created
Refactor invitation and form handler
Add utility view

We need to account for the usecase of initial kickoff mails and add
a basic utility
Merge with upstream