Summary:
Add an setting option to allow automatic dark theme
A feature requested by swrobel https://github.com/facebook/flipper/issues/2708
## Changelog
### UI change
Replace the `Enable Dark Theme` Toggle button with a Radio group containing three Radio buttons.
The available options are `Dark`, `Light` and `Use System Default`, of which `Use System Default` is the default value
<img width="548" alt="Screenshot 2021-08-31 at 3 32 44 PM" src="https://user-images.githubusercontent.com/410850/131462798-4e74f757-41fc-4a3f-ba28-53d00fc1cf03.png">
### Data structure change
The `darkMode` property of [Settings](c0cd32564a/desktop/app/src/reducers/settings.tsx (L20)) is changed from `boolean` to `string`, the available values are `dark`, `light` and `auto`
Pull Request resolved: https://github.com/facebook/flipper/pull/2759
Test Plan:
### Test 1
- Step: Choose `Dark` in the Settings page and click on `Apply and Restart` button
- Expect: The application is displayed in Dark mode
### Test 2
- Step: Choose `Light` in the Settings page and click on `Apply and Restart` button
- Expect: The application is displayed in Light mode
### Test 3
- Step: Choose `Use System Default` in the Settings page and click on `Apply and Restart` button
- Expect: The application is displayed in System Default mode
Reviewed By: mweststrate
Differential Revision: D30666966
Pulled By: passy
fbshipit-source-id: a63e91f2d0dbff96890e267062cb75bffd0007f4
9a4d94c971
