Audit our secondary button uses

[markjaquith]

helen pondered in #wordpress-ui that maybe the "apply" button in the Screen Options dropdown should be a primary button. There's a good argument to be made that when this panel is down, that "apply" button is your new primary.

We should do an audit of our buttons to see if there are any other cases where we could make an action or a temporarily primary action more obvious by using the primary button style.