Fix Uncaught TypeError: (...) unsetLink is not a function

Include existing commands when extending a single command. For example, see
similar example here:

https://tiptap.dev/docs/editor/extensions/custom-extensions/extend-existing#extend-existing-attributes

The full error is:

  TypeError: ctx.editor?.chain(...).focus(...).extendMarkRange(...).unsetLink is not a function

frontend/src/comps/rich-text/extensions/rich-text-editor-link-fix.ts

@@ -4,6 +4,7 @@ import { MultiToken, tokenize } from 'linkifyjs'
 export const CustomLink = Link.extend({
     addCommands() {
         return {
+            ...this.parent?.(),
             /* The tiptap link extension uses a default protocol if a link is
              * entered without a protocol. This does not happen for links entered
              * via the mantine RichTextEditor.Link component (we use in this in