Make "show scores" setting persistent.
The setting is stored in a `post_preview_show_votes` cookie. This means
it's remembered on a per-device basis, but not on a per-account basis.
This is so users without an account can use the setting, and so you can
use different settings on desktop and mobile.
The `view=score` URL param has been replaced by `show_votes=true`. The
`show_votes` URL param overrides the `post_preview_show_votes` cookie.
7976d12cd0