Update black-friday.mjml. Port to V4

[willhertz]

What was done

• Ported to MJML V4

• Removed MOST in-line sytles and moved to attributes and classes. Many of the in-line styles throwed illegal attribute errors.

• Tried to comment as clear and concise as possible for newcomers.

Other Comments:

• Line number fourfolds but markup is cleaner.
• Didn't remove inline style @176
  <mj-button><span style="color:#212020">Shop Now</span></mj-button>

Because
1.  for didactical purposes  (IMHO templates are very good hands-on tutorial for beginners and might be somewhat better than documentation itself)
2.  For some others creating a class for just one style might be an overkill

Environment

Edited and validated using Windows Desktop App MJML App V3.0.2 MKMK v^4.6.2