voting should submit just the item that changed

[ywwg]

this has bugged me forever. The form currently submits the entire form on every change. Instead, the submit function should take the form element name as an argument. The submit function can then read the form and build the post data with a single item. Then the vote function can load the correct proposal, set the single value, and save.

[ywwg]

There is a partial fix to this already -- we pass all of the votes over the POST command, but we only save votes that we detect are different from the database. So at least from a hit-the-db perspective, this is better.