rename google_docs_user to logged_in_user

This is a more generally useful concept. Also remove the check for
controllers responding to a #user method, I didn't find any current
usage of that functionality and it's confusing and dangerous.

refs CNVS-12563

test plan: google docs integration should see no change in behavior,
specifically around not being able to view the docs list of somebody you
are masquerading as.

Change-Id: I6c7f5cd5595970f4ed1782b60431f7529a0024db
Reviewed-on: https://gerrit.instructure.com/33774
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>