Implement a replacement for the Angular JS router which will enable the
root components of the app to be more easily migrated to Preact in future.
Since the app only has three routes with very simple parameters, we
don't yet need a fully-fledged router implementation but can get by with
some pretty basic URL parsing.

 - Introduce a store module which holds the currently active route
   (`null`, "sidebar", "annotation" or "stream") and any parameters of
   that route.

   This state replaces the `isSidebar` boolean in the `viewer` module,
   since whether the app is the sidebar can be determined by checking
   the active route name.

 - Add a `router` service which handles synchronizing the store route
   with the current URL (via the `sync` method or in response to a
   `popstate` DOM event) or updating the URL and active route in the
   store (via the `navigate` method).

 - Modify the `<hypothesis-app>` component to fetch the active route
   from the store and use it to conditionally render the appropriate
   content component. This replaces the Angular router configuration
   that used to be in `src/sidebar/index.js`

 - Change the places that used to read or update the current route to
   read the active route and route params from the store (`store.{route,
   routeParams}`) and switch the current route by using the `router` service
   (`router.navigate`)

 - Remove the unused angular-route package

Fixes #1878