Switch to Ace Editor instead of EpicEditor.

Whiel attempting to fix #90, I found that EpicEditor was adding a bunch of extra newlines to task instructions whenever they were saved. After spending an hour or two attempting to fix this, as well as get around EpicEditor's limitations, I decided to switch to another editor (EpicEditor is also moving away from being a markdown-only editor, so it's long-term usefulness is in question).

Ace Editor seems like a fine substitute. It lacks the preview capability of EpicEditor, but that's of dubious usefulness anyway until we can style it like the site itself. Instead, I think we can add a preview to the site itself at a later point.

The first two commits remove EpicEditor and add django_ace, and the last one contains the actual configuration changes to switch over.