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)