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/widgets/part-1.css

@@ -119,12 +119,11 @@ html.dark .zib-widget {
   color: #fff;
   transition: background 0.2s;
 }
-.profile-social-links a svg,
-.social-icon-link svg {
-  width: 18px;
-  height: 18px;
+.profile-social-links a i,
+.social-icon-link i {
+  font-size: 18px;
+  line-height: 1;
   display: block;
-  fill: currentColor;
 }