[Export] Export data for multiple jobs at once as ZIP

- [ ] Add an option "Download CSV" behind the top level overflow menu next to "Share".
- [ ] The dialog should allow the user to check which layer(s) to export.
- [ ] Clicking "Download" will trigger the download of a zip file containing observations from the selected layers.

In the future, the dialog can be extended to include format (CSV, GeoJSON, WKT), column mappings, export features w/no observations, whether to export features w/o observations, and other filters.