FIX: Various invite system fixes (#13003)

* FIX: Ensure the same email cannot be invited twice

When creating a new invite with a duplicated email, the old invite will
be updated and returned. When updating an invite with a duplicated email
address, an error will be returned.

* FIX: not Ember helper does not exist

* FIX: Sync can_invite_to_forum? and can_invite_to?

The two methods should perform the same basic set of checks, such as
check must_approve_users site setting.

Ideally, one of the methods would call the other one or be merged and
that will happen in the future.

* FIX: Show invite to group if user is group owner

app/controllers/invites_controller.rb

@@ -71,10 +71,6 @@ class InvitesController < ApplicationController
   end
 
   def create
-    if params[:email].present? && Invite.exists?(email: params[:email])
-      return render json: failed_json, status: 422
-    end
-
     if params[:topic_id].present?
       topic = Topic.find_by(id: params[:topic_id])
       raise Discourse::InvalidParameters.new(:topic_id) if topic.blank?
@@ -153,6 +149,12 @@ class InvitesController < ApplicationController
         old_email = invite.email.presence
         new_email = params[:email].presence
 
+        if new_email
+          if Invite.where.not(id: invite.id).find_by(email: new_email.downcase, invited_by_id: current_user.id)&.redeemable?
+            return render_json_error(I18n.t("invite.invite_exists", email: new_email), status: 409)
+          end
+        end
+
         if old_email != new_email
           invite.emailed_status = if new_email && !params[:skip_email]
             Invite.emailed_status_types[:pending]