WIP
oq-ui-api/geonode/exposure/views.py:

Added `validate_export` view to enable a slightly adjusted workflow
requirement in the client.
oq-ui-api/geonode/exposure/tests.py:

Updated some broken tests.
oq-ui-api/geonode/exposure/tests.py:

Added ExportAreaValidTestCase and ValidateExportTestCase.
oq-ui-api/geonode/exposure/urls.py:

Added a route for the `validate_export` view.
oq-ui-client2/src/exposure/exposure_export.js:

Fixed the export/download client workflow to handle error and success
conditions better.

We had to add another server view to enable the desired workflow.
oq-ui-api/geonode/exposure/util.py:

Fixed a pretty serious bug which would cause no results to be exported
if time of day is 'off'.
oq-ui-api/geonode/exposure/tests.py:

Updated tests per a bugfix to the export when time of day is 'off'
oq-ui-client2/src/exposure/exposure_export.js:

Removed an unused variable.
oq-ui-client2/src/exposure/exposure_export.js:

Cleaned up some jqueries, more consistent reference to the
exposure-export-download.
Merge pull request #35 from gem/exposure-export-limit

Exposure export limit

[r=matley] [f=1185841]