add task deletion flow

[acholyn]

At some point we want to allow KW users to delete their tasks - we might want to make this semi difficult (not just click a button and it's gone) - do a "type confirm" box + optionally a reason; seeing reasons for deletion (eg duplication or the study is closed) can influence how we handle it on the db side. We might not want to delete it immediately, maybe a 30 day grace period in which they can undo it?