Update current_election value in _localities

[anlawyer]

The current_election value in https://github.com/caciviclab/odca-jekyll/blob/master/_localities/oakland.md is currently set to June 2018, we should update this value to be the next upcoming election.

Caveat: I noticed that the current_election variable isn't actually being used anywhere in the site (which is probably why we hadn't noticed the wrong date before). I think we should take the time to go through the codebase and find these instances of dead code and remove them. There's no point to having them, and it causes additional confusion to the already somewhat-opaque system on which Jekyll operates.

This could also lead to a discussion about the places that we're maintaining some of the data on the frontend rather than ensuring the backend handles all the variable data.