chore(doc-app): ensure only canonical paths get sent to Google Analytics

Before we were simply sending the current location, but multiple URLs map
to the same document.
Now, we use the canonical path of the current document if available and
fall back to the $location path otherwise.
Includes tests!!

Closes #6402
chore(docs-app): re-activate docs-app test tasks

Currently there is only a minimal test spec in place. But this will now
be run as part of the test tasks.