Fix WCGA and W3C validation issues (iframes)

The html iframes tags that the providers return in the oembed protocol almost always have some acessibility and w3c validation issues.

These issues are:
- WCGA 1.0 and 2.0:
- Iframes tags need title attribute defined to make some screen readers for blind people work.
- As alternative content, you need to add an anchor tag into iframe tag pointing to the content inserted, this way, if the browser or screen reader don't support iframe, the web experience will not be affected by it.
- W3C Validator:
- W3C Validator pointed that frameborder attribute is obsolete, so it needs to be took out.
- Also, looking into W3C Iframe specification, you can see that the attributes allowfullscreen, mozallowfullscreen and webkitallowfullscreen are obsolete too.