Fix "M" hotkey using stale user from fragment cache

Add a SelfAssignmentsController to resolve the current user at request
time, avoiding the use of an incorrect user id baked into a cached URL.

ref: #2211 originally added the "M" hotkey
ref: https://app.fizzy.do/5986089/cards/3722

config/routes.rb

@@ -87,6 +87,7 @@ Rails.application.routes.draw do
       resource :reading
 
       resources :assignments
+      resource :self_assignment, only: :create
       resources :steps
       resources :taggings