LPS-35198 Display number of exportable items

Hey Nate,

I've performed some changes in the ExportImport JS component for this task. Basically:

* I applied @eduardolundgren solution to prevent the dialog buttons to submit the entire form when they are rendered in the form node.
* I changed the name of some methods to adjust them to what they actually do.
* The selected labels are now initialized by the JS component. This way we reduce some duplicated logic and make sure that labels are properly refreshed when the form is reloaded after a change in the date range.
* When the date range is changed, the dialog is reloaded to display the updated numbers. The form it's being submitted to process the date inputs, but export is not performed.

@brianchandotcom These commits include all the frontend logic to display and update the number of items to be exported. As for the backend logic, the general changes in the PortletDataHandlerControl have been performed but they've only been applied to the JournalPortletDataHandler. Once you've validated this pattern, I'll apply it to all other PortletDataHandlers. In the process, we might eventually decide to split or join controls, as we did for Structures, Templates and Feeds for Web content.

@juliocamarero
@LeventeHudak
@MateThurzo