Autofill ballot ID to currently open election?

[rye]

In 30ef630 I add an error if you attempt to access /ballots/new without any election_id param being passed. (either as a JSON body or as a URL query string parameter, Rails doesn't care.)

I wasn't sure if this was optimal. We could autofill the ballot ID to the current election… @erichkauffman, thoughts? Since the case where someone isn't logged in is already handled at the start of this handler, it seems like this makes sense.

[erichkauffman]

Autofilling the ballot ID seems fine since I don't anticipate we will be having multiple elections at the same time.