Create export data dialog

[viddo]

(from dashboard-1.5.sketch)

• Visually should look like in screenshot (see above)
• Add link in the edit (options) menu in the editor
• User can only export a single dataset at a time

[saleiva]

Since this will be needed for the new editor, we can close this isssue for the time being, and reopen once we have this modal implemented.

[viddo]

@saleiva updated description & scope to match current situation:

• Since we agreed to not do batched export to start with I'll removed that from the scope, so will work the same as for current export (single dataset)
• To reach export modal directly form delete dialog is something I consider nice-to-have so put that scope on a separate issue, see Link export modal from delete-datasets modal #3847