Fix modals with lengthy content

[nervetattoo]

Modals would grow out of the document window when a lot of content was added to it. This fix makes it scale in max-height.
It adds the same margin as found on top, to the bottom.

From:

To:

[nervetattoo]

I've gone back and forth a few times before landing on this implementation.
Now it inserts a faux modal in the DOM first in order to measure its top property, then it uses that to set a max-height on the new modal.

Initially I wanted to measure this on the actual modal, but animations make it hard to time the right moment, so this implementation turns out more lightweight.

[circlingthesun]

Vanilla Foundation does not make the modal scrollable but instead lets it expand and appear at the correct position. Have you had a look at #89? I think if you make it scrollable it should be an option instead of default behaviour.

[jbrowning]

Agreed with @circlingthesun. This behavior should be optional. Some apps may prefer to scroll the viewport instead of scrolling the modal content.

[nervetattoo]

You are right. The default behaviour should be the same as in foundation. I was just so damn sure VF made it scrollable inside ;)
I don't think two behaviours should be implemented, so its better to just get #89 merged. Closing this for now.

[jbrowning]

👍 thanks