hgweb: port static file handling to new response API
hgwebdir_mod hasn't received as much porting effort. So we had to
do some minor plumbing to get it to match hgweb_mod and to support
the new response object.
Differential Revision: https://phab.mercurial-scm.org/D2789
hgweb: remove one-off routing for file?style=raw
Now that both functions are using the same API, we can unify how
the command is called and perform command-specific behavior in the
command itself instead of in the high-level router.
Differential Revision: https://phab.mercurial-scm.org/D2788