Fix an issue where the route was set before the initial profile and
groups fetch had completed, which could result in the WebSocket
connection being created and then immediately torn down and re-created
on startup.

The route was set too soon because Safari and Chrome emit a "popstate"
event on page load, which triggered a call to `router.sync()` before the
call to `sync()` made by `setupRoute` in src/sidebar/index.js after the
initial profile and groups load.

The solution is to only add the "popstate" listener after the initial
call to `sync()`.