dialog adapter accepts content and buttons

closes CNVS-14912

test plan:
  - vist new files
  - verify that restriction modal still displays correctly
  - verify that name conflict modal (when adding files) continues to
    work

Conflicts:
	app/coffeescripts/react_files/components/DialogAdapter.coffee

Change-Id: Iedfde2bc15733b3bb90639f2f8bf5582d0f5d139
Reviewed-on: https://gerrit.instructure.com/39640
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>
QA-Review: Jason Madsen <jmadsen@instructure.com>