Make the data export format selector remember your last setting
Depends on D18956. Ref T13049. Make the "Export Format" selector sticky.
This is partly selfish, since it makes testing format changes a bit easier.
It also seems like it's probably a good behavior in general: if you export to Excel once, that's probably what you're going to pick next time.
Test Plan: Exported to excel. Exported again, got excel as the default option.
Reviewed By: amckinley
Maniphest Tasks: T13049
Differential Revision: https://secure.phabricator.com/D18957