Fix display bugs with column picker

[alycejenni]

A list of already selected fields can now be passed to <FieldPicker> components (see #711). The column picker wasn't getting this list so it was throwing an error.

• list of headers/fields now passed into column picker
• default empty list used if no list passed in (this shouldn't happen, but at least it now won't break if it does)
• position and min width set for column picker
• css styles for field picker set for all field picker instances even if not under .term-editor

Fixes: #742