Change the way that links pointing into the Hypothesis service (eg. for
tags, users, account settings etc.) are fetched and generated. The new
implementation better matches patterns used elsewhere in the application
and ensures that UI components displaying these links re-render if the
`/api/links` response is received after the component is initially
rendered.

 - Move the logic for rendering the URL templates from the `serviceUrl`
   service into a selector in the `links` store module. Components then
   render links using `store.getLink(...)`. This ensures use of the existing
   mechanism that re-renders components when relevant store data changes.

 - Convert the `serviceUrl` service to an ES class

 - Move the logic for fetching URL templates from the `/api/links`
   endpoint into an `init` method of the `ServiceURLService` service.
   This matches the convention used by several other services.

 - Remove unused `serviceUrl` dependency from `groups` service