fix tool autoresize to account for canvas footer

test plan:
 - load a page with an external tool
 - upon load, the tool should resize to fill most of the window
 - upon resize, the tool should resize to fill the new window size
   - the first resize should not cause the tool's size to "jump"

Change-Id: I59a7d2ab54f8d2e849b38908d59a91b1be09e8cc
Reviewed-on: https://gerrit.instructure.com/43147
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Mark Severson <markse@instructure.com>