Turn on repo setting: Require sign-off on web-based commits

[a-fox]

Many people making pull requests on the github web interface, and we have sign-off requirement on. To reduce merge errors, we need to enable web-based commit sign-off setting. It's in repo Settings => General. Guide link here.

[a-fox]

I just realised that due to our PR-process, the ToIP repo settings don't affect our commits in forks. Still, it's important to ensure that any commits we do in the main repo will be signed off as well.