Colors rewrite

Created by: MartijnCuppens

Purpose of this PR is to use a uniform approach on using colors:

• Use tint-color() to replace lighten()
• Use shade-color() to replace darken()
• Use scale-color() to replace color-level() (since we don't work with levels anymore)
• Get rid of $theme-color-interval, the 8% levels are hard to calculate with.
• Table variants, list item variants & alerts variants used to have different colors:
  • https://twbs-bootstrap.netlify.app/docs/5.0/content/tables/#variants
  • https://twbs-bootstrap.netlify.app/docs/5.0/components/alerts/#examples
  • https://twbs-bootstrap.netlify.app/docs/5.0/components/list-group/#contextual-classes vs
  • https://deploy-preview-30622--twbs-bootstrap.netlify.app/docs/5.0/content/tables/#variants
  • https://deploy-preview-30622--twbs-bootstrap.netlify.app/docs/5.0/components/alerts/#examples
  • https://deploy-preview-30622--twbs-bootstrap.netlify.app/docs/5.0/components/list-group/#contextual-classes (color contrast on the light variants is still an issue, but can be tackled later on to prevent this PR from being too overwhelming)