Various "Insert Media" dialog tweaks

There's still a lot of frontend work to do around small inconsistencies, for which I've created a separate ticket: 

Note that some of the commits (e.g. the description line height) apply globally in the CMS.

Before

![](http://monosnap.com/image/ousRwDPuIiGLCnGJLd6MNRz5Z.png)
![](http://monosnap.com/image/wl9z6gZ19E0Svj18yemRUmE41.png)
![](http://monosnap.com/image/DT9OW2AiEv3XqhHymEnyEn5iD.png)

After

![](http://monosnap.com/image/Ejofsth8FJcCsLSgm8q19N6pq.png)
![](http://monosnap.com/image/mkw5uYgk3qeTKOmH3GwZ9i7sx.png)
![](http://monosnap.com/image/Hrdc6LNCif1O4quulEdpovMv0.png)