Rewrite site header and fiddle with related stuff.

  - Rewrite site header markup and related styles.
  - Refactor navigation module to be more general-purpose.
  - Remove old and unnecessary code from "scss/main.scss".
Refactor and tweak various things.

  - Remove shadows from under post body, metadata, and notes.
  - Remove unnecessary import statements from "scss/main.scss".
  - Remove responsive design stuff, for now.
  - Rename a couple of variables and reorganise "scss/variables.scss".
Change how length measurements are calculated.

Change length-related values so that they are defined in pixels, which
are easier to work with, but convert them to ems or percentages when
compiling.