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/specific/forum.scss

@@ -13,7 +13,7 @@ div#c-forum-topics {
 
     &.label {
       display: inline-block;
-      font-size: 0.7em;
+      font-size: var(--text-xs);
       text-transform: uppercase;
       border: 1px solid;
       border-radius: 0.25rem;

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

@@ -12,7 +12,7 @@ div#c-moderator-dashboards {
   }
 
   caption {
-    font-size: 1.1em;
+    font-size: var(--text-lg);
     font-weight: bold;
     text-align: left;
   }

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

@@ -65,7 +65,7 @@
 
 div#c-posts {
   .fav-buttons {
-    font-size: 14pt;
+    font-size: var(--text-lg);
 
     button.ui-button {
       padding: 0.25em 0.75em;

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

@@ -112,7 +112,7 @@ div#c-uploads {
     }
 
     .caption-top {
-      font-size: 0.8em;
+      font-size: var(--text-sm);
       margin-bottom: 0;
     }
   }

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

@@ -14,7 +14,7 @@
       position: absolute;
       top: 0.5rem;
       right: 0.5rem;
-      font-size: 2em;
+      font-size: var(--text-xxl);
     }
 
     nav#nav {