Verification # labeled `tracking` in api/unlocked-votes json

[dsernst]

This is an artifact from when the feature now called Verification # used to be called Tracking #.

We renamed it on the front-end, but never on the backend.

Now that we're mirroring the unlocked-votes.json to GitHub, for HACK SIV post-election verification, it would be good if we had this more consistent.

• The unlock-votes route is the one setting this name, when it's stitching the partials back together.

• The front-end <DecryptedVotesTable /> component is the one currently looking for tracking, that will need to now look for verification.

• Will still want to have the front-end support renaming the old tracking field name too, to maintain backwards compatibility with previously created elections.