Add confirmation dialog for unselecting winners

[struan]

Ask if people want to unset the winners and include count of how many will be unset to make sure they understand it's all winners

Fixes #935

[mhl]

Looks good :+1: Just as a note, after the merge we'll need to do the steps in https://github.com/mysociety/yournextrepresentative/blob/master/docs/transifex.md - in particular not forgetting the makemessages -d djangojs step.