[F#] Update instruction screens to F#.

When the F# apps are run the first time, the user gets to the instructions screen with some C# stuff still on it.

- [x] Updated iOS instruction images: F# code snippet and `LoginViewController.fs` filename (original and @2x)
- [x] Ran new iOS images through ImageOptim shrinking.
- [x] Updated Android HTML text with F# code.

Note: Android already had the correct filename (`LoginFragment.fs`).