Confirm before deleting collection
Confirm before bulk deletes too
Merge branch 'feature/confirm-delete' of https://github.com/diracdeltas/securedrop into diracdeltas-feature/confirm-delete
replace inline event handlers

they won't work with the CSP that we set by default in production (and are not
best practice)

more informative confirmation messages

ignores nonsensical request to delete multiple collections if no collections
are selected