Introduce mobile beta feature tests

We register a new driver called `mobile` where we emulate a mobile
viewport. We also renamed the default driver to `desktop`.

To run the tests for mobile, set the environment variable. For example:
`CAPYBARA_DRIVER=mobile rspec spec/features/beta/webui/example_spec.rb`

Co-authored-by: Eduardo Navarro <enavarro@suse.com>