improve handling of logout issues

refs CNVS-14595

quit pinging the server with requests that will fail after you've
been logged out

test plan:
 * enable page views
 * log in, and go to an assignment page. enable request monitoring
 * be constantly moving your mouse while on this page
 * notice a request to /ping every 3 minutes, and /page_views every
   5
 * stop your server
 * go back and continue moving your mouse
 * requests to /ping and /page_views should fail, but continue
 * restart your server
 * requests should now succeed
 * in another browser, or an incognito window, log in as the same
   user, then log out
 * in the first window, the request to /ping should fail with
   a 401, and the request to /page_views should fail with a 422
 * continue moving your mouse - requests should stop

note that it's easiest to reduce the 3/5 minute times by adjusting
them in app/coffeescripts/behaviors/ping.coffee and
public/javascripts/page_views.js

Change-Id: Id558504f940af51df8bb410d1d88668cd95fdb38
Reviewed-on: https://gerrit.instructure.com/38733
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>