Fix HTML tags in topic titles.

We no longer sanitize titles before saving to the database since it
would cause problems like HTML entities showing up when you try to edit
a topic title. It isn't even really necessary since we only render
fancy_title directly and never title.

The escaping logic used here is the same that is used both in lodash and
onebox. See:

1. https://github.com/discourse/onebox/pull/190/files
2. https://github.com/lodash/lodash/blob/2.4.1/dist/lodash.compat.js#L6194