Standardize color keyword arguments in components

[joemull]

Description or user story

Steph has come up with a set of utility classes, currently living in button.css, that allow a component to take color information from the caller and reliably produce accessible contrasts. The way it's structured encourages using only the colors we have established in the scheme, because both foreground and background color are used in the class.

Proposed solution

I think we should move this to utilities.css, get rid of best_foreground, and refactor the page section component (and others, though I think it's just the George splash?) to use this.

We should also consider whether the border color can be set with this utility, since it is especially useful in our style for button borders to have the same color as text.

Alternatives

Keeping the current two solutions in the codebase and getting confused between them.

[joemull]

In design we discussed systematizing Steph's foreground+background classes as discussed above, but we decided against including border-color in these classes, for now. Later we may want to add a mechanism for accessing a "pop color" relative to the applied background.