Add sign-up tests for mobile, minor adjustments to other mobile tests

Created sign-up tests for the mobile version to address #5168. Amended and added to the getting started tests.

There appear currently to be no alerts or flash messages as part of sign-up or getting-started in the mobile version, so these tests are truncated, and I'm not sure how useful they will be. But they are there, and they pass.

I have also repositioned `I toggle the mobile view` in some other tests so all actions, where possible, are done in the mobile version.

(Also re-arranged the items in `mobile_steps.rb` and made a couple of updates to one desktop test file.)