Fix frontpage avatars (#13853)

The frontpage uses a rather strange method to obtain the commit's avatar
which I've overlooked earlier. I don't exactly understand how it works
but this change fixes the wrong default avatars by using the function
that was in previous use.

Also introduced a few constants for size an size increase factor.

Fixes: https://github.com/go-gitea/gitea/issues/13844

web_src/less/_base.less

@@ -312,6 +312,12 @@ a.muted:hover,
   font-weight: normal;
 }
 
+/* fix misaligned images in webhook dropdown */
+.ui.dropdown .menu > .item > img {
+  margin-top: -.25rem;
+  margin-bottom: -.25rem;
+}
+
 .ui.selection.dropdown .menu > .item {
   border-color: var(--color-secondary);
 }
@@ -1101,16 +1107,6 @@ footer {
   text-align: center;
 }
 
-.generate-img(16);
-.generate-img(@n, @i: 1) when (@i =< @n) {
-  .img-@{i} {
-    width: (2px * @i) !important;
-    height: (2px * @i) !important;
-  }
-
-  .generate-img(@n, (@i + 1));
-}
-
 // Conditional display
 @media @mediaMdAndUp {
   .mobile-only,
@@ -1660,6 +1656,11 @@ a.ui.basic.label:hover {
   visibility: hidden;
 }
 
+/* prevent stacking context issue on webhook dropdown */
+.ui.segment {
+  position: static;
+}
+
 .ui.segment,
 .ui.segments,
 .ui.attached.segment {