Rework diary entry comments

[gravitystorm]

Diary entry comments are separate from diary entries, and it makes sense to split them into different controllers to work on the given model.

However, there's one little gotcha that I found out. Comments are always created from the diary entry page, and when the comment is invalid (e.g. blank) the same diary entry is rendered, in order to show the error messages. This is quite hard to do from diary_comment#create or similar, so it needs some thought as to what to do.