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.

app/javascript/src/styles/common/utilities.scss

@@ -28,6 +28,8 @@ $spacer: 0.25rem; /* 4px */
   white-space: nowrap;
 }
 
+.whitespace-nowrap { white-space: nowrap; }
+
 .leading-none { line-height: 1; }
 
 .absolute { position: absolute; }
@@ -45,6 +47,7 @@ $spacer: 0.25rem; /* 4px */
 .mx-0\.5 { margin-left: 0.5 * $spacer; margin-right: 0.5 * $spacer; }
 .mx-2    { margin-left: 2   * $spacer; margin-right: 2   * $spacer; }
 
+.mt-1  { margin-top: 1 * $spacer; }
 .mt-2  { margin-top: 2 * $spacer; }
 .mt-4  { margin-top: 4 * $spacer; }
 .mt-8  { margin-top: 8 * $spacer; }

app/javascript/src/styles/specific/z_responsive.scss

@@ -41,11 +41,13 @@
     }
   }
 
-  #posts #posts-container {
-    width: 100%;
-    display: flex;
-    flex-wrap: wrap;
-    align-items: center;
-    justify-content: flex-start;
+  .posts-container {
+    display: grid;
+    grid-template-columns: repeat(3, minmax(0, 1fr));
+    gap: 0.25rem;
+
+    &.user-disable-cropped-false article.post-preview img.has-cropped-true {
+      object-fit: none;
+    }
   }
 }