FEATURE: add category banner for why a user cannot post (#9576)

* FEATURE: add category banner for why a user cannot post

Adds a category banner for why a user is unable to post in a category.

Also adds an extra alert for the user when a user is unable to create a topic in a
category and they still try and click on the disabled-looking new topic
button.

db/migrate/20200427222624_add_read_only_to_categories.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddReadOnlyToCategories < ActiveRecord::Migration[6.0]
+  def change
+    add_column :categories, :read_only_banner, :string
+  end
+end