fix facebook iframe layout css

also clarify some facebook plugin config steps

fixes CNVS-13203

test plan:
- go to <canvas>/facebook/ and make sure the page renders

Change-Id: I5efdcbd16c497533e5aeee8b29ee59b57c8deb98
Reviewed-on: https://gerrit.instructure.com/35174
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>