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

@@ -18,6 +18,7 @@ $spacer: 0.25rem; /* 4px */
 .text-muted { color: var(--muted-text-color); }
 
 .text-xs { font-size: var(--text-xs); }
+.text-sm { font-size: var(--text-sm); }
 
 .leading-none { line-height: 1; }