Fix modals with lengthy content

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:
![screen shot 2014-08-25 at 10 10 15](https://cloud.githubusercontent.com/assets/45449/4027483/68fd306e-2c2f-11e4-8a6e-095ae93164a6.png)

To:
![screen shot 2014-08-25 at 10 10 32](https://cloud.githubusercontent.com/assets/45449/4027482/68fa822e-2c2f-11e4-8b3b-98c661791cba.png)