add screenreader alerts in cross-listing section dialog

when entering a course id in the course section cross-listing
dialog, the results should be read by the screenreader.

also fixed the styling of the dialog buttons to match the
standard canvas dialog button styling.

fixes CNVS-14007

test plan:
 - go to the course section cross-listing dialog.
 - enter a bogus course ID and press enter.
 - screenreader should read both the intermediate "confirming"
   message and the final error message.
   - the confirming message might get cut off by the error
     message in some screenreaders, that's just how they work.
 - enter a valid course ID and press enter.
 - screenreader should read both the intermediate "confirming"
   message and the name of the course.
   - the confirming message might get cut off by the name of
     the course in some screenreaders, that's just how they
     work.
 - make sure both the "cross-list this section" and "cancel"
   buttons still work.

Change-Id: I645f769285148cb4e40e2e92f7542aad04ed4c50
Reviewed-on: https://gerrit.instructure.com/39093
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>