Make the data export format selector remember your last setting

Summary:
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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18957