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

`get_exposure_export_form` now accepts only GET requests instead of POST.

POST was not appropriate.
oq-ui-client2/src/exposure/exposure_export.js:

Make the bounding box lon/lat vars global.

We may want to display these vars somewhere else in the exposure app,
so this makes them easier to access.
oq-ui-client2/src/exposure/exposure_export.js:

Changed the exposure export form load request from a POST to a GET.

We were using jQuery.load() before, but if you pass parameters to it,
it it automatically treated as a POST (lame!). Unfortunately,
jQuery.get() doesn't support partial document loads the way load()
does, so I had to basically add the equivalent code.