Completely re-reading the security book

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

Well, this should be interesting :). Several years ago, I bootstrapped the security chapter and it's been there ever since. That fact doesn't necessarily mean that it was good. I've just re-read and basically re-written the chapter from scratch. I thought it was too long, too theoretical in the beginning, and it also had some extra "baggage" just from being old and having things added to it.

My goal is to:

A) Not actually remove anything of importance - I've done my best with this
B) Actually get feedback that this is better. I feel that this is better, but rewrites aren't automatically better. It's like the second album of a band - even though they're older and wiser, maybe the original is still better :). I hope not!

Todo:

- [ ] fill in config blocks - @xabbuh if you happen to have some time and can help, I would be even more in debt to you :)

As I merge to 2.5 and up, I'll need to check for the `versionadded` tags on each branch and re-add those things to the new chapter.

Thanks!
@iltar we have another cookbook for that, but yes, this is a pain, so we need to make sure it's both (a) clear enough in that cookbook (I think it is now, but not 100% sure) and that (b) there isn't a way to find some other page on the docs and miss all that info about `Serializable`.

In this case, I don't think people looking for how to hook up their security system with their database will find this page as a starting point, so I hope it's not an issue. But more practically, I think there are only 2 built-in providers, so I just showed those 2.

But on that note, this example has always seemed like crap to me because you'll end up with 2 different user classes floating around depending on who logged in. And this means that you could only ever really use the `UserInterface` methods safely around your app.

So, is this even a decent example? Do we know something better?