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;

assets/css/frontend/pages/base-1.css

@@ -205,11 +205,10 @@ html.dark .archive-timeline__badge {
   color: var(--color-ink-3);
   transition: border-color 0.2s, color 0.2s;
 }
-.oauth-btn svg {
-  width: 22px;
-  height: 22px;
+.oauth-btn i {
+  font-size: 22px;
+  line-height: 1;
   display: block;
-  fill: currentColor;
 }
 
 .oauth-btn:hover {

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;
 }