hgweb: load globally-enabled extensions explicitly
Before, extensions were loaded as a side effect of hg.repository() if the
hgweb was executed as a CGI/WSGI. I want to make it explicit so that another
ui hook can be inserted after extensions.loadall().
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}