refactor(icons): replace local brand SVGs with FA5 classes

Unify social/OAuth/contact icons on the global Font Awesome 5.15.4 pack,
add aksrt_render_social_links for author hero, enqueue FA on admin SPA,
and delete assets/img/icons.

assets/css/frontend/footer/zibll-shell.css

@@ -144,17 +144,10 @@
   background: var(--focus-color, var(--color-primary));
 }
 
-.footer .footer-contact > a i,
-.footer .footer-contact > a svg {
+.footer .footer-contact > a i {
   vertical-align: middle;
 }
 
-.footer .footer-contact > a svg {
-  width: 1em;
-  height: 1em;
-  fill: currentColor;
-}
-
 .footer .footer-wechat-img {
   display: none;
   position: absolute;