Back to Ballot link should specify the election or ballot returned

[DaleMcGrew]

Please describe the issue (What happens? What do you expect?)

If I send a candidate link to a friend, the "Back to Ballot" link should take me to the election that the candidate is in, and it doesn't currently.

Steps to reproduce the problem (1, 2, 3...), including links

1. On WeVote.US choose any election except Alabama
2. Go to this candidate in the Alabama race: https://wevote.us/candidate/wv02cand33006/b/btdb/
3. The Back to ballot takes you to the non-Alabama race, and it should.
   Solution: When you are on a candidate page with "/b/btdb/" (back to default ballot) at the end, make the "Back to Ballot" button take the voter to this url (with the "1111" as the google_civic_election_id): /ballot/election/1111
   OR /ballot/id/:ballot_returned_we_vote_id

If you are on a candidate page with "/btvg/" (back to voter guide) like this: https://wevote.us/candidate/wv02cand30772/btvg/wv02org145
...then take them back to:
/voterguide/:organization_we_vote_id/ballot/election/:google_civic_election_id
OR /voterguide/:organization_we_vote_id/ballot/id/:ballot_returned_we_vote_id

[DaleMcGrew]

Excellent work @lazorfuzz! Verified.