Summary:
As designed by Vince [here](https://www.figma.com/file/4e6BMdm2SuZ1L7FSuOPQVC/Flipper?node-id=585%3A127550), this diff adds Flipper Doctor into Sandy
Note:
- The dot on Doctor icon will act similarly to `DoctorBar`
- add type to `count` for `LeftRailButton` to act like dot badge
- Get rid of padding in `antd` modal
Reviewed By: nikoant, mweststrate
Differential Revision: D24137349
fbshipit-source-id: 8ce441e0ed96083eba09d98dfd3a45ff9b5be027
2b6bac4227