Remove masthead and intro wrappers

I expect to need wrapper variants in the future, but these ones were
not doing much of anything.
Move <section> styles to classes for reusablity
Indent intro__bio class
Bump up the padding on sections
Increase size of the main grid

Add a compact grid for sections that need less columns
Remove Dash object
Change .intro colors, and set variables for them
Tweak .avatar box-shadow
Revise section component's description
Make 'section' component an object

It has no thematic styling. It doesn't belong as a component.
Make a note about .avatar's height and width
Remove Noto Serif
Only use Roboto Slab on the masthead, sans-serif on other headers
Allow more arguments in the dash mixin
Add a dash to the intro section
Align font stacks
Reorder classes and add an ID to the intro section