added submitter handle to PR

that included:
- recording it in session, on auth success
- retrieving it on pr submission

also:
- moved signed_in to application_helper
- minor cleanup of session creation
- dried up user name/handle access
- added func to create github acct url from handle
- fixed error (s/below/above) from moving a link
added submitter handle to PR

that included:
- recording it in session, on auth success
- retrieving it on pr submission

also:
- moved signed_in to application_helper
- minor cleanup of session creation
- dried up user name/handle access
- added func to create github acct url from handle
- fixed error (s/below/above) from moving a link