FEATURE: Various improvements to invite system (#12023)

The user interface has been reorganized to show email and link invites
in the same screen. Staff has more control over creating and updating
invites. Bulk invite has also been improved with better explanations.

On the server side, many code paths for email and link invites have
been merged to avoid duplicated logic. The API returns better responses
with more appropriate HTTP status codes.

lib/wizard/builder.rb

@@ -276,10 +276,10 @@ class Wizard
             users = JSON.parse(updater.fields[:invite_list])
 
             users.each do |u|
-              args = {}
+              args = { email: u['email'] }
               args[:moderator] = true if u['role'] == 'moderator'
               begin
-                Invite.create_invite_by_email(u['email'], @wizard.user, args)
+                Invite.generate(@wizard.user, args)
               rescue => e
                 updater.errors.add(:invite_list, e.message.concat("<br>"))
               end