Allow students to delete threads / respones; prepare for edit view for response comments (but not complete)

Fix staff markers for inline threads and double staff markers for threads with deeply nested comments.

Extra spaces actually get added to a post when activating a WMD editor which can
get saved into the post if you click update.

Make appended_id for WMD editor a little less hacky by getting it from data-id of the 'reply-body' element instead of from _id of the wrapper.