DEV: deprecate `invite.via_email` in favor of `invite.emailed_status`
This commit adds a new column `emailed_status` in `invites` table for
tracking email sending status.
0 - not required
1 - pending
2 - bulk pending
3 - sending
4 - sent
For normal email invites, invite record is created with emailed_status
set to 'pending'.
When bulk invites are sent invite record is created with emailed_status
set to 'bulk pending'.
For invites that generates link, invite record is created with
emailed_status set to 'not required'.
When invite email is in queue emailed_status is updated to 'sending'
Once the email is sent via `InviteEmail` job the invite emailed_status
is updated to 'sent'.
