Add Task-wielding, Blob-backed bulk export handler
Poll using AJAX to download CSVs from bulk exporter
name GAE version export-bulk
pollForCSVDownload(): switch AJAX GET to POST to handle large id lists
Bulk download js: switch the right AJAX call to POST and check statuses correctly
export_bulk_handler: increase sites queried per task from 1 to 100
export_bulk_handler: optimise by checking Event id
Bulk download js: slow down polling to every 2 sec
Download CSV: show wait message
Reapply temporary version name of 'export-bulk'
Main map: show spinner gif when exporting CSV
Main map: replace gif spinner with js spinner
Merge branch 'export-bulk'

Task-wielding, Blob-backed bulk export handler and associated web
front-end.