Automated response: Code is not defensive about disabled content hooks

If a content hook has been modified to be disabled, or somehow is naturally disabled yet other code is using it when it should not be, Composr should be a lot more fault tolerant.

I'm not going to post a hotfix as I changed so many files here.

(I could not reproduce the original bug report without editing the wiki_page content hook's code manually. But I think this fix would solve the issue.)