stick new activity button to the right place
stick new activity button to the right place

requires the new activity commit from the canvas-planner: FALCOR-366

the new activity button isn't sticky in IE yet, so it should just slide
under the header.

closes FALCOR-371

test plan:
* have new activity and scroll down in Canvas
* the new activity button should stick to the bottom of the header
  in supported browsers
* the new activity button should slide under the header in IE

Change-Id: I022e0319ef3482d71f27a38b9b266edfdbbb9a9a
Reviewed-on: https://gerrit.instructure.com/116240
Tested-by: Jenkins
Reviewed-by: Dan Minkevitch <2591e5f46f28d303f9dc027d475a5c60d8dea17a@instructure.com>
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Jon Willesen <0321efbc0acf6204eb6035acb1d418838b450d08@instructure.com>

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