Add dropdowns to `inbox` view; adjust CSS to more closely match mockup.
Back out `overflow: hidden` on `.filters`, as it hides the dropdown menus.

We need to reparent the dropdown menus on open if we want to hide overflowing
dropdown menus using `overflow: hidden`. Another option would be to dynamically
adjust the width/visibility of the dropdown menus via script. Leaving it this
way for now; work in progress.