templater: move stylemap() to hgweb_mod, since that's its only user
`stylemap()` even has an error message that mentions "hgweb
templates", so it seems that it's meant specifically for hgweb.
Differential Revision: https://phab.mercurial-scm.org/D8890
#require docutils gettext $ $TESTDIR/check-gendoc it checking for parse errors