Add a very basic custom_css property to a conference.

It is loaded on all pages for that conference.

db/migrate/20200715001812_add_custom_css_to_confereneces.rb

@@ -0,0 +1,5 @@
+class AddCustomCssToConfereneces < ActiveRecord::Migration[5.2]
+  def change
+    add_column :conferences, :custom_css, :text
+  end
+end