added default front page contents

this reinstates much of the infrastructure first introduced in
74b611b6d6c8a0e0af1ca33a72123b7fd95f54a0, but later removed in favor of
properly static assets (cf. e6fb82055dcc853ba75c17b38691c62963487d01)

initially this seemed like a lot of black magic again, as the process
(ibuilder reads `instance:store_contents` to create package store,
imaker uses those generated contents to populate the instance store) is
not very intuitive, but now it makes sense - for the moment anyway...

note that the test suite's app initialization desperately needs
review/refactoring - not least because the dev instance's
`tiddlywebconfig.py` interferes with testing

using "bfw" as bag owner to indicate its special status as a system bag

/cc @cdent