Batch discussion/post operations

We want to allow multiple discussions/posts to be selected, and then an action to be performed on all of them at once.

What needs to be done:
- Design the UX for this. My initial thought is that there could be a "select" option in the discussion/post controls menu, and when clicked, all of the controls menus for other discussions/posts turn into checkboxes. The action buttons could then be displayed in a box floating somewhere on the page?
- Work out implementation on the front-end. It will likely involve objects similar to DiscussionControls and PostControls.
- Work out implementation on the API-end. It will likely involve the [JSON-API Bulk extension](http://jsonapi.org/extensions/bulk/).