Add a documentation upload form

This adds a simple page for uploading and deleting package documentation. It uses some fancy transform trickery to re-use the existing REST API (kudos to @dcoutts for the idea!).

Currently the "delete documentation" option isn't terribly useful, since the build client always ignores packages that have failed before. It would be nice to have a real "force rebuild" button, though I'm not sure how that'll work.