Uploads for admin site settings (#4913)

* Improve OpenGraph tags for about pages

* Add thumbnail admin setting

* Fix error

* Fix up

db/migrate/20170913000752_create_site_uploads.rb

@@ -0,0 +1,10 @@
+class CreateSiteUploads < ActiveRecord::Migration[5.1]
+  def change
+    create_table :site_uploads do |t|
+      t.string :var, default: '', null: false, index: { unique: true }
+      t.attachment :file
+      t.json :meta
+      t.timestamps
+    end
+  end
+end