Enable 'visualize' button for script fields in Discover

Closes #2201 in several ways. 

- Normalize visualize buttons. Gets rid of the red/yellow/green button in favor of calming dark blue button.
- Replaces them with a contextual tooltip and a warning count depending on the features of the field.
- Changes the filter buttons to use the new .filterable property.
- Allows visualization for fields that are not indexed, but are scripted.

![image](https://cloud.githubusercontent.com/assets/1250387/5368460/bb255ef6-7fc3-11e4-98e8-2f812757de2e.png)