Add markdown cache columns to the database, but don't use them yet

This commit adds a number of _html columns and, with the exception of Note,
starts updating them whenever the content of their partner fields changes.

Note has a collision with the note_html attr_accessor; that will be fixed later

A background worker for clearing these cache columns is also introduced - use
`rake cache:clear` to set it off. You can clear the database or Redis caches
separately by running `rake cache:clear:db` or `rake cache:clear:redis`,
respectively.

1 files changed, 4 insertions, 0 deletions

diff --git a/app/models/appearance.rb b/app/models/appearance.rb
index 4cf8dd9a8ce..e4106e1c2e9 100644
--- a/app/models/appearance.rb
+++ b/app/models/appearance.rb
@@ -1,4 +1,8 @@
 class Appearance < ActiveRecord::Base
+  include CacheMarkdownField
+
+  cache_markdown_field :description
+
   validates :title,       presence: true
   validates :description, presence: true
   validates :logo,        file_size: { maximum: 1.megabyte }