[rtl] restore auto-flipping of old arrow icons
[rtl] restore auto-flipping of old arrow icons

Fixes: CNVS-43278

The InstUI icons have Start-* and End-* variants that will flip for us
But the old ones that we use classnames to make still need to be flipped
In rtl. We had this, but I took mistakenly it out in g/149482 when I 
also took the stuff for the instUI ones

Test plan:
* turn on RTL
* go to /calendar
* the arrow icons on top should look like this:
  https://cl.ly/ac78ae2020de
  And not this:
  https://cl.ly/c949c7f053c1

Change-Id: I94526906a25070e0a17321a82a243cfddb85dbcc
Reviewed-on: https://gerrit.instructure.com/169820
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Tested-by: Jenkins
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms