try to algin jetpack form checkbox with sensei.

[unprintedch]

#2221 - try to align jetpack form checkbox with sensei.

• Added a hook for custom class in jetpack form fields (wporg-learn-form)
• Styled it

Open questions:

How could we make a good use of sensei css variables? I use the SVG icon in the CSS, I find it bad... (and the linter too) can someone tell me how to do it properly? And where to put this "checkbox.scss" it is an override of a vendor, any good practice?