[#1251] Embed view uses absolute URL in iframe
[#1251] Adds frameBorder="0" to the resource view's embed iframe

It's not valid HTML5, but there're no better ways so far AFAIK.
[#1251] Change embed code's help text

Copied from OpenSpending.
[#1251] Embed code now has configurable width/height

I also moved the CSS to the right place: the .less files.
[#1251] Don't try to center the modal div

The problem is that we were trying to get the element's height when it was
still hidden (display: none). This always return 0px.

There're a few hacks, or I could've moved the div when it was actually shown,
but I didn't feel it was worth it. I don't think it's that bad to have it show
at the top of the page.
[#1251] Style embed modal's width and height inputs