add full_url to tabs api json

test plan:
* using the tabs api, confirm it returns a full_url

closes #CNVS-1737

Change-Id: I655cf06ab2819ac6ab079b8f2767c2b5333556b0
Reviewed-on: https://gerrit.instructure.com/41689
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>