Enable App Group capability

[UInt2048]

This PR simply enables the app group capability, a requirement for future PRs such as for a widget.

If App Groups aren't enabled for your provisioning profile in App Store Connect, you'll need to do so.

[davidsansome]

It looks like I can't register the group.com.davidsansome.wanikani app group identifier - perhaps because it's been registered already. I don't suppose you registered it yourself while testing? Can you check at https://developer.apple.com/account/resources/identifiers/list/applicationGroup?

Anyway, I've decided to stop accepting pull requests for major features (like widgets) from now on. The ones that've been merged recently have taken quite a lot of extra polish to get them up to a level of quality that I'm happy with, and there are others (like the watchkit integration) that I know I'm going to need to get back to and clean up eventually. I feel like they're adding a pretty big maintenance burden on me, and blocking me from doing releases until I'm happy the new features are up to scratch.

[UInt2048]

I understand. I am so sorry that I have made you feel this way. I didn't realize the burden that I was putting on you.

I would have stopped earlier had I known that I would ruin it for everyone. I promise not to provide major PRs to you in the future nor create accounts for the purpose of making PRs.

I hope you might consider lifting that ban if someone else in the future is able to provide a higher release quality.