Separate Google Analytics events of Girder web app and external web app

[brianhelba]

The google_analytics plugin doesn't currently distinguish the root URL from the Girder web client and the web_external web client. For example, navigating to '/' and '/girder' will both result in a hit to '/' on Google Analytics. As another example, /girder/collection/... URLs show up as /collection/..., i.e. the /girder prefix is missing.