Unify dropdown button styles

Unifies dropdown styles across app (use one shade of gray for border, always use caret instead of chevron)   
Before:
![Screen_Shot_2016-11-02_at_1.39.20_PM](/uploads/f0a2808c97aaa404626631683da32380/Screen_Shot_2016-11-02_at_1.39.20_PM.png)
After:   
![Screen_Shot_2016-11-02_at_1.39.45_PM](/uploads/cca0d40138d48f11cfc7183b202abad9/Screen_Shot_2016-11-02_at_1.39.45_PM.png)

Should affect _all_ dropdowns
![Screen_Shot_2016-11-02_at_1.35.12_PM](/uploads/73874a46d79144a7a854eab57030c2bc/Screen_Shot_2016-11-02_at_1.35.12_PM.png)

See merge request !7250