hgweb: don't responsd to api requests unless feature is enabled
Per discussion at https://phab.mercurial-scm.org/D2834, new URLs
in hgweb can conflict with subrepos and virtual repos. This may prevent
access to repos in certain paths or having certain names.
Until we have a workaround for this, let's not serve requests for
"api/" URLs unless the feature is enabled.
Differential Revision: https://phab.mercurial-scm.org/D2936