Addon api

[mitchelljkotler]

• Adds a filter on categories. Expects a categories key in the top level of the config, with a list of categories as strings. It is not enforcing a set of approved categories at the moment.
• Renders markdown description to HTML in place on config update. Doing it in place means we would need to manually update all add-ons on deploy (not a big deal, just need to remember to do it), as well as update the frontend and backend in sync.