Remove mentions of INVITED state

fixes #2142 (closed)

See issue comment https://gitlab.com/gitlab-org/gitter/webapp/issues/2142#note_176630486 for detailed explanation of the problem this MR is solving.

• Remove all references of user.state being 'INVITED'
• STRETCH GOAL: remove mentions of invitedToRoom, invitedByUser, inviteReminderSent which are part of the same removed logic
• Remove the one user with state DISABLED from the production database
  • Remove references in the code
• ??? Remove all users that were created as a part of the original invite logic but never signed up

Test strategy

For sending emails I've used debug logging in mailer service (thanks @MadLittleMods) DEBUG=gitter:mailer npm start

1. • Existing user can be invited to a room
   1. • existing user will get notified of being invited to a room
2. Non-existing user can be invited to Gitter when email is provided
   1. • user will receive email
   2. • user can accept invite
3. • Removed user will still have "not active" UI treatment on Gitter
   •