Add copy-to-clipboard button to meta page

Fixes #42.

assets/js/copyclipboard.js

@@ -0,0 +1,39 @@
+"use strict";
+
+const COPY_TO_CLIPBOARD_STATUS_ID = "copyToClipboardStatus";
+let copyToClipboardCtr = 0;
+
+function copyToClipboardSuccess() {
+    let statusElem = document.getElementById(COPY_TO_CLIPBOARD_STATUS_ID);
+    statusElem.innerText = "URL copied!";
+    statusElem.classList.remove("fail");
+    statusElem.classList.add("success");
+}
+
+function copyToClipboardFail(cause) {
+    let statusElem = document.getElementById(COPY_TO_CLIPBOARD_STATUS_ID);
+    if (!cause) cause = "unknown error";
+    statusElem.innerText = "copy failed: " + cause;
+    statusElem.classList.remove("success");
+    statusElem.classList.add("fail");
+}
+
+function copyToClipboard(url) {
+    let copyEventIdx = ++copyToClipboardCtr;
+
+    if (!window.navigator.clipboard) {
+        let msg = "could not access clipboard";
+        if (window.location.protocol !== "https:") {
+            msg += ": website not using HTTPS"
+        }
+        copyToClipboardFail(msg);
+        return;
+    }
+    window.navigator.clipboard.writeText(url).then(() => {
+        if (copyToClipboardCtr !== copyEventIdx) return;
+        copyToClipboardSuccess();
+    }, () => {
+        if (copyToClipboardCtr !== copyEventIdx) return;
+        copyToClipboardFail();
+    });
+}