Move parsoid-specific code out of the pagecontent bucket handler

For ease of review, I tried to keep each commit atomic (and non-breaking) so that the progress of the refactor is captured.

There's still some cleanup needed, but all the code migration should be done.

This will somewhat deprecate #46, since it will be used to support the cache-control header for parsoid-generated data.  This PR does not address the general pattern of cache-control header support on all endpoints.
Speaking of which.. we really need to change Parsoid's return to something that gives us the content-type. That type includes the spec version, so is important not to make up. Will follow up in https://phabricator.wikimedia.org/T75955.