Mobile: different checkout progress indicator needed

[KFie]

Environment: mobile

The checkout progress indicator needs to be different on mobile devices as it wraps onto two lines and takes up too much space. Please check if this will still be true with resolution of #189.

Possible solutions if issue persists:

• only display the numbers without step name and decrease the bubble icons so they all fit on one line
• use meaningful icons instead of simply numbers (which would then also apply to desktop)

[freddyka]

Issue not fixed for poster v2.1.

-->reason is that mobile menu button can't be used (see#199)