css: standardize font sizes.

Fix various elements to use standard font sizes instead of ad-hoc sizes.

Noticeable changes:

* Tags in autocomplete are slightly smaller.
* The favorite heart icon on posts is slightly smaller.
* Pool titles on thumbnails in the pool gallery page are slightly bigger.
* The page footer is slightly smaller.
* Timestamps on comments and forum posts are very slightly smaller.
* "Pending"/"approved"/"rejected" labels on forum posts are very slightly smaller.

app/components/post_preview_component/post_preview_component.scss

@@ -30,7 +30,7 @@ article.post-preview {
   }
 
   .desc {
-    font-size: 80%;
+    font-size: var(--text-sm);
     margin-bottom: 0;
   }