Refactor _buttons & add interaction-accent support
Refactor _buttons & add interaction-accent support

There was an issue with contrast on the dark theme for certain ui
elements mainly outlined ones (e.g. outline-button, icons, links, etc.),
specifically:
- interaction-norm on the dark bg (e.g. outlined button)
- white text on interaction-norm (e.g.  solid button)

The issue is lightening or darkening one use-case makes the other worse,
therefore design saw a need to introduce another concept to interaction
colors which is interaction-accent (going to be used for anything
outlined essentially)

CP-2421

ProtonMail/WebClient,ProtonMail/WebClient,ProtonMail/WebClient