Example-forum newsletter uses inconsistent cookie methods

[georgezee]

In commit 6e78509, the cookie handling was refactored to use 'universal-cookie'.
See:

However the function to update the cookie wasn't changed, which seems a simple oversight. It still uses:
Cookie.save('showBanner', 'no');
rather than:
cookies.set('showBanner', 'no');

Note, I haven't tested this (yet), but it seems straightforward. Will raise a PR shortly.