Allow "unvoting"

[p-offtermatt]

We should offer a way for users to "unvote", i.e. remove their existing vote for a proposal, but without putting up a vote for another at the same time.

In the code, this would mean refactoring the "vote" method into something like unvote and vote:

• unvote deletes a previous vote for a given lockup
• vote assumes that the lockup has not yet voted in this round/tranche, and creates the vote according to the input