Add read-only states for components

[bendansby]

This state supports a tooltip and is focusable by users, but is otherwise unresponsive to input. Read more here: https://paper.dropbox.com/doc/UI-Style-Guide-and-Caveats--Bp4fqW9iugIrl0HvFqdDzOlbAg-PvOLxeX3oyujYEzdJx5pu#:uid=089559839007265064583003&h2=:under-construction:-Read-Only

The following components need read-only states

• [ ] Button
• [ ] Select
• [ ] Text input -- note that this component already has a "read-only" state—it should be preserved but renamed "selectable"
• [ ] Toggle
• [ ] Checkbox
• [ ] Radio button
• [ ] Text area