FIX: Allow add email to group if user can invite (#13097)

It used to allow adding email addresses to a group even if invites were
disabled for the site. This does not allow user to input email address
if they cannot invite.

The second thing this commit improves is the message that is displayed
to the user when they hit the invite rate limit.

app/controllers/groups_controller.rb

@@ -368,7 +368,15 @@ class GroupsController < ApplicationController
       end
 
       emails.each do |email|
-        Invite.generate(current_user, email: email, group_ids: [group.id])
+        begin
+          Invite.generate(current_user, email: email, group_ids: [group.id])
+        rescue RateLimiter::LimitExceeded => e
+          return render_json_error(I18n.t(
+            "invite.rate_limit",
+            count: SiteSetting.max_invites_per_day,
+            time_left: e.time_left
+          ))
+        end
       end
 
       render json: success_json.merge!(