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/messages.scss

@@ -21,7 +21,7 @@ div.list-of-messages {
 
       a.message-timestamp {
         font-style: italic;
-        font-size: 0.90em;
+        font-size: var(--text-sm);
         color: var(--muted-text-color);
         &:hover { text-decoration: underline; }
       }
@@ -54,7 +54,7 @@ div.list-of-messages {
         a.message-timestamp {
           display: inline;
           color: var(--muted-text-color);
-          font-size: 0.8em;
+          font-size: var(--text-sm);
         }
       }
     }

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

@@ -3,7 +3,7 @@ div#news-updates {
   background: var(--news-updates-background);
   border-bottom: 2px solid var(--news-updates-border-color);
   overflow: hidden;
-  font-size: 0.8em;
+  font-size: var(--text-sm);
 
   ul {
     float: left;

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

@@ -53,7 +53,7 @@ form.simple_form {
         display: block;
         color: var(--form-input-validation-error-text-color);
         font-style: italic;
-        font-size: 0.8em;
+        font-size: var(--text-sm);
       }
     }
 

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; }