Invitation email enhancements - added org name and domain name to
subject and replaced the "contact the sender" text at bottom of body
(card
https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1194087).
Achieved by customising django-organizations' default InvitationBackend
and supplying alternative templates (activated by new setting
INVITATION_BACKEND).
Reference inviting user in invitation email by username (mandatory) instead of first_name
and last_name (optional so could be empty).
Use username instead of first_name and last_name in reminder emails
(username is mandatory, first_name and last_name not, so can be blank).
Kanban card
https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1201979.
Changed text in reminder email body - kanban card
https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1194105
Merge pull request #78 from jogwen/invitation_email_fixes

Invitation/Reminder email fixes