removed static assets from store

the original idea (cf. 74b611b6d6c8a0e0af1ca33a72123b7fd95f54a0) was for
static assets to reside within the package (as pkgstore), being linked
into instances via diststore

however, that not only added a fair amount of complexity, it also makes
it difficult or impossible for static assets to be served by the web
server rather than the application

furthermore, it's questionable whether application assets should be
treated as tiddlers / user-generated content - getting rid of this
special bag also means we don't have to worry about that distinction
anymore (cf. 9fb7efc629e8621199477f17f50312ca88f63a0d,
fe7370daa8d5408506191addfa1675953ecc879c,
0636a335ac24691dcc8f52c9c7b476f5f873b3c0)

thus in upcoming commits we'll treat static assets as regular files
rather than tiddlers

note that the adjustments in `config.py` and `instance.py` rely on
yet-unreleased changes to imaker:
https://github.com/cdent/tiddlywebplugins.imaker/pull/3
added handling for static assets

as expected in the previous commit
(e6fb82055dcc853ba75c17b38691c62963487d01), static assets are now
treated as regular files instead of tiddlers

there are two kinds of assets: local and remote - the former reside in
the package's "assets" directory, the latter are reference in
"remote_assets" and copied into the former directory using a simple
script (`assetr`)

these assets are served via an application handler which retrieves them
directly from the package

caveats:
* no caching headers yet (though this is true for all handlers at the
  moment)
* ideally, static assets should be served by the web server rather than
  the application - for this a separate script will need to copy all
  assets into a predefined directory because the web server cannot
  evaluate package resources at runtime
* mixing local and remote assets in the same directory makes it awkward
  to distinguish between both (cf. the Makefile's `clean` target, but
  more problematic in `.gitignore`)
updated Makefile to use null terminators for safety

this avoids errors for file names with spaces (e.g. within the
virtualenv)
moved and renamed remote assets directory

as pointed out by @cdent, these don't need to be part of the package:
https://github.com/FND/tiddlywebplugins.bfw/pull/19#discussion_r5534834

not sure whether "externals" is a suitable name though
replaced static handler with generic plugin

NB:
* this relies on a yet-unreleased version of tiddlywebplugins.static:
  https://github.com/tiddlyweb/tiddlywebplugins.static/pull/1
* tiddlywebplugins.static is being initialized from within BFW's
  initialization to ensure it takes precedence over subsequent BFW
  handlers - it would be nice if this could be replaced by a simple
  entry in `system_plugins`, but I got myself confused between `config`
  and `instance_config` (the former doesn't seem right while the latter
  doesn't seem to do anything)
removed obsolete dependency reference

this was forgotten in e6fb82055dcc853ba75c17b38691c62963487d01
ensured tests use the spawned instance

this not only ensures that the instance works as expected, it also
simplifies the setup considerably

caveats:

* not sure why the "templates" symlink is required
* it appears the instance's `tiddlywebconfig.py` is not being evaluated;
  within `load_app` the settings from `instance.instance_config` (e.g.
  `server_host`, `system_plugins`) are no longer present, even when
  forcing a `merge_config` beforehand

/cc @cdent
ensured test instance's `tiddlywebconfig.py` is loaded

thus resolving the config issue mentioned in the previous commit
(56eef3ec19bd2203b5c232bbbe87a65cf2bdb975)

note that this required properly calculating the respective cookie as
`CONFIG['secret']` is no longer predictable
moved static initialization into config

now possible thanks to proper test instance (cf. previous commits;
56eef3ec19bd2203b5c232bbbe87a65cf2bdb975 and
bdfc20db812b0ff3182c58e0ed2fab8da96944a1)
Merge branch 'reasset'