Invitation for onboarding new people

[tedyoung]

• [x] 1. Adjust security for /invite needing to be authN
• [x] 2. TEST: Create database table for invitations: token, username, date_created, date_used, was_user
• [x] 3. TEST: InvitationRepository has methods: containsActiveInviteFor(invite_token, username); markInviteAsUsed(token)
• [x] 4. TEST: NewMemberInvitationController: need AuthPrincipal and "invite_token" request param
• [x] 5. TEST: Need failure page for invite not valid
• [x] 6. Add username constraint to database