Introduce \OCA\Mail\Service\Html

:construction: 
You need to specify the sandbox attribute: 
`<iframe src="{{htmlBodyUrl}}" sandbox seamless>` 

Additionally, this URL should be not guessable. Elsewhere an attacker might send you a vulnerable mail and then send you a link to it. In this case the protection offered by the `sandbox` attribute will be gone. I'd solve this by just doing a CSRF check here and include it into the URL.

Btw: If you don't care about other browsers than WebKit it might be easier to use the `srcdoc` attribute since we save a HTTP request and don't need to implement much more, e.g.:
```html
<iframe sandbox seamless
        srcdoc="<p>This is a mail, javascript won't work in-here!</p><script>alert(1)</script>"></iframe>
```

That said: The sandbox attribute will per default tell the browser to handle this iframe as would it be served from another origin. From a security perspective this is awesome, but in our case (if we don't use `srcdoc`) this is impracticable since we need a way to authenticate the user and the token. Implementing a URL based authentication layer (e.g. by unique tokens) seems overkill for that to me.
I'd therefore recommend to add `allow-same-origin` to the sandbox attribute: `<iframe src="{{htmlBodyUrl}}" sandbox="allow-same-origin" seamless>`. If we do not enable Javascript with (`allow-scripts`) it shouldn't change much from a security PoV.

Have a look at http://www.html5rocks.com/en/tutorials/security/sandboxed-iframes/ for a nice write-up about the sandbox attribute.