[RB] 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
I would do

```
data = util._get_currency_and_taxonomy_name(sr_id, occupancy)
if data:
    currency, taxonomy_name, taxonomy_version = data
else:
    ...
```