Moves geoserver-specific url routes to the geoserver app.

**What does this PR do?**
Moves the `/layers/acls`, `/layers/resolve_user` and `layers/download` URL routes to the `geonode.geoserver` application. 

**Backwards compatibility.**
For now, the old URL routes (using the `/layer` prefix) still work, but should be removed once legacy code is updated. 

**Issues**
None.