Refactor CSS

[querkmachine]

Closes #64 and #52.

Changes

• Rewrites Sass so that most variables exist as CSS custom properties instead of Sass variables.
  • Refactors most everything else to use these properties.
• Rewrites Sass to use logical properties and values (again).
• Makes the website a utopia (without changing it too much):
  • Redefines the typography scale to follow utopian scaling principles.
  • Redefines the grid size and gutters to follow utopian scaling principles.
  • Redefines the spacing classes to follow utopian scaling principles.
  • Adds new spacing classes that go 'one-up' on larger breakpoints.
• Adds a new 'meta' component for post/page metadata.
• Adds container queries!
  • Adds a new container query Sass mixin.
  • Refactors the media query mixin so that common logic lives in the same function.
  • Masthead and figure components now use container queries when determining changing things up.
• Alters the spacing classes used on the wrapping elements of most pages.
• Fixes up some instances of code being unnecessarily duplicated:
  • Updates blog cards to use the kimLink class for links.
  • Updates colour scheme switcher to use kim-!-sr class for visually-hidden inputs.
• Fixes up some instances of things looking a bit nasty without CSS:
  • Avatars on Mastodon shares, favourites and comments now have maximum image dimensions in HTML.
• Various small design tweaks:
  • Table of content text is now larger.
  • Some previously bolded headings are now not bold.
  • Headings in footer and above interactions are slightly smaller.
• Reorganises most of the settings and helpers.
• Removes most of the Sass helpers.
• Reclasses skiplink component to be an object.
• Updates postcss and postcss-preset-env dependencies.
• Updates colophon page to reference Utopia.