Fix "score" UI

[eevee]

• When you're not logged in, there's a frankly gigantic message telling you that you need to log in to rate art. I'd rather just not show the controls; most guests won't even know that's a thing.
• The score is shown as a floating-point number with a zillion decimal places, which sucks.
• I'd really rather not show the score at all, at least by default. Show it to admins, show it to artists if they opt in somewhere, but otherwise, don't make this a popularity contest. The number of ratings is a nice statistic (and also discourages mass downvoting), so let's put that there instead.
• Consider changing the symbols to adjectives (bad, eh, good?) instead; may or may not encourage more use.

[epii]

93e7ffb and 2f04e91 should fix the first three points from the issue description, but the whole thing still needs going over from a UI POV.

FWIW, I actually like the up/down arrows, though the Circle Of Eh is not obviously part of the scale and needs to be... maybe made larger? Changed to a big dash?

Also, while a user may change their rating there is currently no method by which to revoke/undo a rating altogether. Should that be added? If someone fat-fingers it or their opinions change then an ability to set "no-vote" may be more meaningful than leaving them to vote "eh". Presumably you'd unvote by clicking the currently selected rating?

[eevee]

Cool beans, though needs a migration.

I don't really like the idea of revoking a rating; I mean, the intention here (and the most useful outcome) is for everyone to rate every piece s/he sees. If you decide you don't actually think the art is all that great, okay, just rate it neutral.