tweak webish to use resource_filename to find css and html files

tweak webish to use resource_filename to find css and html files

using sibpath to find web template files relative to source code is functional
when running from source environments, but not especially flexible when running
from bundled built environments.  the more 'orthodox' mechanism, pkg_resources,
in theory at least, knows how to find resource files in various environments.

this makes the 'web' directory in allmydata into an actual allmydata.web module
(since pkg_resources looks for files relative to a named module, and that module
must be importable) and uses pkg_resources.resource_filename to find the files
therein.