Add view for importing batch of tasks

Conflicts:
	oneanddone/tasks/templates/tasks/form.html
Save task batch to database
Add confirmation step to import form

Conflicts:
	oneanddone/tasks/templates/tasks/form.html
Add import confirmation with sessions. Clean up display

Conflicts:
	oneanddone/tasks/templates/tasks/form.html
Import 20 fresh bugs at a time
Make invalidation criteria formset dynamic
Clean up
Address review comments
Enable config. of field names for invalid. criteria
Display bug summary in import-confirmation

Conflicts:
	oneanddone/tasks/templates/tasks/form.html
Move criterion-formset functions into site.js
Accept both BMO REST and search-results URLs
Display number of tasks in confirmation
Replace out-of-date migration
Address second set of review comments