The behavior of `GroupsService.load` varies depending on whether the current
route is the sidebar or not. During sidebar startup, `GroupsService.load` was
called before `RouterService.sync`. As a result the route was `null` during the
initial call to `GroupsService._loadGroupsForUserAndDocument` and hence the
service did not execute the sidebar-specific code path to wait for the document
URI to become known at that point.

During the first `GroupsService._loadGroupsForUserAndDocument` call,
`_setupAutoReload` would set up a watcher that would react to changes in
`store.mainURI()`. This watcher would fire after the main guest frame connects.
Depending on when that happens, requests from the initial `GroupsService.load`
request could still be in-flight. If the second set of groups API calls
completed first, followed by the first set of groups API calls, then the final
set of loaded groups might reflect the _first_ call to `GroupsService.load`,
where the document URI was not used (because the route was unknown).

This commit fixes the issue by re-arranging the sidebar startup sequence to
initialize the router service before other services.

Fixes https://github.com/hypothesis/support/issues/79