[6.x] Forms 2: Allow choosing which columns to export

[duncanmcclean]

This pull request implements the ability to choose which columns are included when exporting form submissions.

Previously, the export modal let you pick a format and whether to export all or filtered submissions, but every export included every field. Now there's a "Columns" section in the modal, with all fields (plus the date) selected by default and a "Select All" / "Deselect All" toggle.

Column selection lives in the modal, rather than on individual exporters, since it's a property of the request in the same way filtering is. The selected columns are passed to the exporter, which decides how to honour them.

Exporters

The base Exporter class gains a columns() helper which returns the selected column handles in form order, defaulting to every field plus date when nothing has been selected. The built-in CSV and JSON exporters use it, and custom exporters can too.

Custom exporters with a fixed schema can opt out of column selection by overriding supportsColumnSelection() to return false, which hides the "Columns" section when that exporter is chosen.

This PR also extracts the export modal into its own ExportSubmissionsModal component.

Before

After