returned to tiddler API for checklists

exposing the tiddler model is not a bad thing for BFW - quite the
contrary, it's desirable at some level

plus there are practical concerns: using our own primitive handlers
reinvents the wheel while missing out on things like ETags (including
edit conflict protection)

This partially reverts commit 6a8bc04d03f7dea57cc7d47e5d3b573b11e7137b.
introduced rel=canonical link pointing to tiddler URI

not sure whether canonical is the right type, but it's good enough for
now