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.

lib/rate_limiter/limit_exceeded.rb

@@ -11,8 +11,8 @@ class RateLimiter
       @type = type
     end
 
-    def description
-      time_left =
+    def time_left
+      @time_left ||=
         if @available_in <= 3
           I18n.t("rate_limiter.short_time")
         elsif @available_in < 1.minute.to_i
@@ -22,7 +22,9 @@ class RateLimiter
         else
           I18n.t("rate_limiter.hours", count: (@available_in / 1.hour.to_i))
         end
+    end
 
+    def description
       if @type.present?
         type_key = @type.tr("-", "_")
         msg = I18n.t("rate_limiter.by_type.#{type_key}", time_left: time_left, default: "")