Select: Fix hardcoded SCSS items

[benjamincharity]

https://github.com/GetTerminus/terminus-ui/blob/master/terminus-ui/select/src/select.component.scss#L122-L126

• padding should be spacing(small, 1)
• right should be -#{spacing(large, 1)}
• panel-header should be added to the z-index map above menu in _z-index.scss
• _z-index.scss should be added to imports
• z-index should be z(panel-header)