R1:dd94512837d7

Abstract and further merge filter menus

Summary:
- Adds `PhabricatorMenuItemView` which is a non-hacky object representing a single menu item.
- Adds `PhabricatorMenuView`, a collection of items.
- Deletes some busted/old interfaces full of garbage nonsense.
- Merges menu item styles from `aphront-side-nav-view-css` and `phabricator-nav-view-css`. These are old-style and new-style rules which got partially updated recently.
- The new-style menus have a darker background (#ececec) than the old…