refs https://github.com/TryGhost/Team/issues/928
- we want to make use of the same color adjustments and contrast selection for accent colors we use in Admin on the server-side for emails so utility functions have been extracted to an external package
refs https://github.com/TryGhost/Team/issues/928
- switched to using `color` for color conversion and adjustments rather than maintaining our own limited utils (preparation to extract our own utils to separate library)
- changed contrast threshold for yiq-based contrast adjustment from `128` to `186` to match Portal's current behaviour
refs https://github.com/TryGhost/Team/issues/927
- added CTA `button and url inputs to email-cta card
- added `textColorForBackgroundColor` color util and used it to add a white/black text color variable that can be used when the accent color is used as a background color
- added `{{hex-adjust}}` helper for modifying lightness and saturation of a hex color
- adjusted inline power-select dropdown styling
