Some templates can legitimately contain invalid HTML (e.g. mismatched tags)
because the templates operate in pairs.

In these instances we should not use an editor because the nature of the
editor (content editable) means that the browser automatically corrects all
HTML supplied to it, thus breaking the template entirely.

Therefore we need to disable HTML editors for some templates, and do so in
a way tied to the instance of the activity, rather than to a specific user.

This patch adds a new 'config' field, with matching setters and getters, to
allow such per-instance values to be stored.