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: https://github.com/silverstripe/silverstripe-framework/issues/2100

Note that some of the commits (e.g. the description line height) apply globally in the CMS.

Before

![](https://api.monosnap.com/image/download?id=ousRwDPuIiGLCnGJLd6MNRz5Z)
![](https://api.monosnap.com/image/download?id=wl9z6gZ19E0Svj18yemRUmE41)
![](https://api.monosnap.com/image/download?id=DT9OW2AiEv3XqhHymEnyEn5iD)

After

![](https://api.monosnap.com/image/download?id=Ejofsth8FJcCsLSgm8q19N6pq)
![](https://api.monosnap.com/image/download?id=mkw5uYgk3qeTKOmH3GwZ9i7sx)
![](https://api.monosnap.com/image/download?id=Hrdc6LNCif1O4quulEdpovMv0)