Exposure export ui changes

This PR includes commits from the ged-add-api-to-list-countries-and-subnational-regions branch. The PR is a complete refactor of the exposure export tool.
The new version of the exposure export tool includes a completely new approach that includes a new workflow and new API endpoints.

View the application help page for information about how that application should be used: http://www.globalquakemodel.org/openquake/support/documentation/platform/help/exposure/

Please note that the help page link '?' is not going to be working until Pr 294 is merged into master