[copy link] improved 'copy link to the clipboard' (#4419)

* copy link to the clipboard, and make it shown as a link when pasted
into the seafile markdown editor

frontend/src/components/copy-to-clipboard.js

@@ -0,0 +1,45 @@
+import deselectCurrent from 'toggle-selection';
+
+function copy(text) {
+  let reselectPrevious, range, selection, mark, success = false;
+  try {
+    reselectPrevious = deselectCurrent();
+
+    range = document.createRange();
+    selection = document.getSelection();
+
+    // 'a' is for 'copy a link to seafile markdown editor'
+    mark = document.createElement('a');
+    mark.textContent = text;
+    
+    document.body.appendChild(mark);
+
+    range.selectNode(mark);
+    selection.addRange(range);
+
+    const successful = document.execCommand('copy');
+    if (!successful) {
+      //console.log('copy command was unsuccessful');
+    }
+    success = true;
+  } catch (err) {
+    // console.error('unable to copy using execCommand');
+  } finally {
+    if (selection) {
+      if (typeof selection.removeRange == 'function') {
+        selection.removeRange(range);
+      } else {
+        selection.removeAllRanges();
+      }
+    }
+
+    if (mark) {
+      document.body.removeChild(mark);
+    }
+    reselectPrevious();
+  }
+
+  return success;
+}
+
+export default copy;