Add cancel button to mash dialog

Could temporarily get from page, then if cancel is hit do a Mash.load with that temporarily saved data to cancel.

Start at the mash dialog and the Mash.load entry point mentioned in the issue. Trace how the page obtains its temporary data, then verify that cancel restores that saved data and that the dialog visibly provides a cancel button; no test file is named in the issue.

Written by the indexing model from the issue text.