enabled checklists' checkboxes for direct interaction

by default, markdown-checklist's checkboxes are disabled to avoid
falsely suggesting interactivity

this proof of concept adds that required interactivity via JavaScript

caveats:
* using TiddlyWeb URIs seems a bit wrong - mostly because this way the
  client has to know about tiddlers' JSON structure
* eventually the JavaScript code should be refactored, with generic
  parts being moved into markdown-checklist:
  https://github.com/FND/markdown-checklist/issues/1
simplified checkbox interactivity

cf. previous commit 80d54377ccd36f02483b66c4ead00f9568a40c49

instead of relying on the tiddler model, we're now making the wiki page
handler either return or accept the respective source as plain text

review from @cdent would be appreciated; not sure whether this new logic
belongs into the `wiki_page` handler (also see inline comments)
refactored client-side checklist augmentation

... splitting it into a generic and an application-specific module -
which (pending documentation) should make it ready to be moved into
markdown-checklist
removed unused import

this was forgotten in 4727ec35e516989491683ffc3fdb870ae9b0713d
moved JavaScript module into markdown_checklist

this required extending the assetr mechanism to support package
references
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=edit link pointing to tiddler URI

not sure whether this is the right type - it seems more appropriate than
rel=canonical anyway - but it's good enough for now
Merge branch 'checklists'