Move custom javascript into custom.js

- May be affected by compressor layout

assets/js/custom.js

@@ -0,0 +1,12 @@
+window.onload = () => {
+    let commit = document.querySelector("#commit-id > a");
+    if (commit != null) {
+        let colorCode = commit.innerHTML.trim().substring(1, 7);
+        commit.addEventListener('mouseover', function handler() {
+            commit.style.color = `#${colorCode}`;
+        })
+        commit.addEventListener('mouseout', function handler() {
+            commit.style.color = '';
+        })
+    }
+}