Display notification that max concurrent sessions has been reached (GRN-41) (#317)

* <Added max concurrent sessions check>

* <Refactored some exception logic>

* <Fixed code style>

* <Generalized BBBException error message>

app/controllers/rooms_controller.rb

@@ -94,7 +94,11 @@ class RoomsController < ApplicationController
     opts = default_meeting_options
     opts[:user_is_moderator] = true
 
-    redirect_to @room.join_path(current_user.name, opts, current_user.uid)
+    begin
+      redirect_to @room.join_path(current_user.name, opts, current_user.uid)
+    rescue BigBlueButton::BigBlueButtonException => exc
+      redirect_to room_path, notice: I18n.t(exc.key.to_s.underscore, default: I18n.t("bigbluebutton_exception"))
+    end
 
     # Notify users that the room has started.
     # Delay 5 seconds to allow for server start, although the request will retry until it succeeds.