more admin dashboard polish, some code cleanup

admin/static/releases.js

@@ -0,0 +1,25 @@
+document.addEventListener('readystatechange', () => {
+    const newReleaseBtn = document.getElementById("create-release");
+    if (newReleaseBtn) newReleaseBtn.addEventListener("click", event => {
+        event.preventDefault();
+        const id = prompt("Enter an ID for this release:");
+        if (id == null || id == "") return;
+
+        fetch("/api/v1/music", {
+            method: "POST",
+            headers: { "Content-Type": "application/json" },
+            body: JSON.stringify({id})
+        }).then(res => {
+            if (res.ok) location = "/admin/releases/" + id;
+            else {
+                res.text().then(err => {
+                    alert(err);
+                    console.error(err);
+                });
+            }
+        }).catch(err => {
+            alert("Failed to create release. Check the console for details.");
+            console.error(err);
+        });
+    });
+});