Fix CodeMirror retaining history of unrelated notes (#1640)

ts/editor/NoteEditor.svelte

@@ -147,6 +147,11 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
 
     let noteId: number | null = null;
     export function setNoteId(ntid: number): void {
+        // TODO this is a hack, because it requires the NoteEditor to know implementation details of the PlainTextInput.
+        // It should be refactored once we work on our own Undo stack
+        for (const pi of plainTextInputs) {
+            pi.api.getEditor().clearHistory();
+        }
         noteId = ntid;
     }