Remove HTML Purifier

We're no longer using it, having switched to Tidy. (As wonderful as
HTML Purifier is.)
Make rewrites reflect current language, system.

Modernize language, simplify structure.
Fix "formatted" field for general dictionary terms

Per #269, fix the "formatted" field to pull data from the proper
location, and also store it within the same numbered object, rather
than outside of it.
Establish a Varnish class and a purge method

Per #171.
Fork off part of url_to_structure() to get_current()

Per #264.
Correct URL parsing mistake
Store the Varnish host's URL in a constant

Per #171.
Merge remote-tracking branch 'upstream/master'