FEATURE: send max 200 emails every minute for bulk invites (#7875)

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'.

db/migrate/20190711154946_add_emailed_status_to_invite.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class AddEmailedStatusToInvite < ActiveRecord::Migration[5.2]
+  def change
+    add_column :invites, :emailed_status, :integer
+    add_index :invites, :emailed_status
+
+    DB.exec <<~SQL
+      UPDATE invites
+      SET emailed_status = 0
+      WHERE via_email = false
+    SQL
+  end
+end