chrome: give a service its own navigation row

The switcher lists the instance's services, each on an origin of its own.
Three artifacts pages rode ExtraNav into it, so that row's membership
changed from service to service and its active element - spent naming
"artifacts" - said nothing about which of the three the reader was on.

Sections is the second row instead, the one upstream draws as
.header-tabbed wrapping .nav.nav-tabs and meta puts profile and keys in.
A service declares its sections at startup; Page marks the one the
request's path stands in, so a page cannot forget to declare itself, and
a path in no section lights nothing rather than the first tab. Both
classes arrive with core.sr.ht's nav.scss, so the row costs no service
any CSS.

ExtraNav is deprecated with it: bench and cov abandoned it when the
instance deployed a tokens.sr.ht and the word appeared in the navbar
twice, and artifacts was its last user.