[native] Add horizontal margin to buttons in LoggedOutModal

Summary:
In order to iterate on the new registration flow, I'm going to replace the register button with two side-by-side register buttons, with the second one only visible to devs.

In order to make it easier to add space between those side-by-side buttons, I'm adding some horizontal margin to the buttons. (This wouldn't be necessary on React Native 0.71, where we could use gap properties instead of margin ones.)

Test Plan:

before	after

The only difference is in the horizontal margins of LoggedOutStaffInfo

Reviewers: ginsu, atul

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7494