Reorganize CSS files (#24739)

Reorganize various CSS files for clarity, group together by subdirectory
in `index.css`. This reorders some of the rules, but I don't think it
should introduce any issues because of that.

web_src/css/repo/list-header.css

@@ -0,0 +1,36 @@
+.list-header {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: .5rem;
+}
+
+.list-header-sort {
+  display: flex;
+  align-items: center;
+  justify-content: flex-end;
+  padding-left: 1rem;
+  padding-right: 1rem;
+}
+
+.list-header-search {
+  display: flex;
+  flex: 1;
+  align-items: center;
+  flex-wrap: wrap;
+  justify-content: center;
+  min-width: 200px; /* to enable flexbox wrapping on mobile */
+}
+
+.list-header-search .input {
+  flex: 1;
+}
+
+.small-menu-items {
+  min-height: 35.4px !important; /* match .small.button in height */
+}
+
+.small-menu-items .item {
+  padding-top: 6px !important;
+  padding-bottom: 6px !important;
+}