Enforce signatures on post/patch operations

[carletex]

Fixes #44

I worked on this with @dgrcode and I've just gave it the final touches.

This PR enforces the use of signatures on challenge / challenge review submissions. Not sure if we are missing something with the signatures, but I think this should be secure enough.

Let me know what you think.

[dgrcode]

I'll address all the comments besides the middleware and commit. I'll also open a new issue for the middleware improvement.

[dgrcode]

I made a few changes related to the comments I added before. Just merged into master a few hours before dogfooding, which is probably not recommendable, although it'd be awesome to show this feature today.

@sogasg Could you please give it a run in your local before dogfooding? I checked everything works on my machine, but this things are mysterious some times, especially on live demos.

In case things blow up, the commit master had as HEAD before merging was 2af7e33. Feel free to
git reset --hard 2af7e33b8ba20c1d049a02f71c051a9b6c447ce8
if needed. I hope this is not necessary! 🤞

[carletex]

Thanks @dgrcode !!

Just tested locally and everything seems to work.

@sogasg Good luck at the dogfooding, let us know how it goes.

[asgeir-s]

Awesome work!

Everything is working locally for me too :D