Produce a screenshot catalog of the app

Screenshot testing can be used to ensure that there is no regression in the UI of the app, but automated screenshot pixel comparisons are hard to maintain and keep reliable.

However, there's also value for the screenshots of every screen of the app without automated testing:

- Be able to manually compare the end result with screens from the original designs easily
- Be able to go back in time and see older versions of a screen without re-installing old APKs
- Be able to have an already existing screenshot to edit and draw on to suggest future changes
- Be able to perform manual screenshot comparisons at key moments - a side by side set of images would be easy to identify regressions visually.

We have a UIScreenSanityTest which walks the app with the robot and ensures each page does not crash. We could provide a screenshot catalog by extending this to take a screenshot at each point in the walk.

This could perhaps be saved and stored for current develop and each release of the app.

cc @nadonomy who I was talking to about this idea just now.