Allow election admins to bulk upload ballots

[ArendPeter]

Long term this will be a necessary feature for elections with paper ballots.

Short term this will be helpful for testing.

We can reuse the logic that was written for bettervoting.com/UploadElections tool.

Tasks

• Add an upload ballots section to the https://bettervoting.com/<election_id>/admin/ballots page (similar to bettervoting.com/UploadElections)
• Add a checkbox in the settings for allowing admins to upload ballots. This should be disabled by default, but admins can enable it prior to the election