votes: fixup various minor issues.

* Add a gap between thumbnails on mobile.
* Adjust CSS for scores and vote buttons.
* Include "Private favorites" as an incentive on the user upgrade page.
* Fix vote buttons not being visible beneath thumbnails on mobile.
* Fix the "Show scores" link not preserving the current page number.
* Fix vote buttons being unintentionally enabled for all thumbnails by default.
* Fix banned and restricted users being able to favorite posts by
  tagging them with `fav:self`.
* Fix search engines being able to crawl /posts?view=score pages.
* Fix broken tests.

test/factories/favorite.rb

@@ -2,5 +2,9 @@ FactoryBot.define do
   factory(:favorite) do
     user
     post
+
+    factory(:private_favorite) do
+      user factory: :gold_user, enable_private_favorites: true
+    end
   end
 end