Onboarding Welcome step - markup

[m4dz]

This PR introduce markup and style for the first step (welcome) of new onboarding.

• [x] static markup and poc style
• [ ] report static markup in app views
• [ ] create dedicated stylesheet for this step (related to #315 )

[m4dz]

Just a little doubt about the CSS class name "cozy"

Yeah, me too. I'll try to find something more explicit (dunno at this time)

[m4dz]

Oops, just seen this one is comming for the wrong branch in my repo. Closing and reopen it soon.