Reputation log: better record inversed votes

Currently, voting down after voting up (or the opposite) just removes the old vote and replaces it with the new one, recording only one `UserAction` for the new vote. To make sure we register things properly with the reputation, we should record a `revert_vote` action before recording the new one.

No file or test is named. Start by tracing the vote handling and UserAction recording mentioned in the issue, then verify that changing an existing vote records a revert_vote action followed by the new vote action.

Written by the indexing model from the issue text.