Refactor repo header/list (#29969)

1. Use general "mobile-only" and "not-mobile" CSS styles, remove some`@media (max-width: 767.98px)` tricks
2. Use `CountFmt` for repo list, just like the repo header (and it matches GitHub, to avoid big numbers bloat the page)

web_src/css/repo/header.css

@@ -1,4 +1,4 @@
-.fork-flag {
+.header-wrapper .fork-flag {
   margin-top: 0.5rem;
   font-size: 12px;
 }
@@ -15,11 +15,8 @@
   padding: 0;
 }
 
-.repo-header .btn.interact-bg:hover {
-  text-decoration: none;
-}
-
 .repo-header .flex-item-main {
+  flex: 0;
   flex-basis: unset;
 }
 
@@ -27,10 +24,6 @@
   flex-wrap: nowrap;
 }
 
-.repo-header .flex-item-trailing .repo-icon {
-  display: none;
-}
-
 .repo-buttons {
   align-items: center;
   display: flex;
@@ -75,15 +68,3 @@
   padding-top: 12px;
   background-color: var(--color-header-wrapper);
 }
-
-@media (max-width: 767.98px) {
-  .repo-header .flex-item {
-    flex-grow: 1;
-  }
-  .repo-header .flex-item-trailing .label {
-    display: none;
-  }
-  .repo-header .flex-item-trailing .repo-icon {
-    display: initial;
-  }
-}