Scripted fields interface

Adds interface to create, edit and remove scripted fields from the index pattern.

- New interface lives in settings/indices in a new tab called "Scripted Fields"
- Button to add new scripted fields
- Interface to create an edit scripted fields
  - Includes error message when using a name that matches an existing indexed field

Also adds another change to the `paginated-table` directive - columns can now be marked as not sortable.

This completes the second task on #1537 
I realize that this probably works fine, but an explicit check for `undefined` is dangerous IMO. The scripted property is boolean and should probably be treated as `true` or `false`-y, not `true` or `undefined`.