stack post and comments when width < 500px

The goal is to dispose of the dedicated `narrow' mode and
to do all style adjustments via @media queries.
wrap narrow.scss styles in @media query

Mostly indentation changes. It's now safe to always
load the narrow stylesheet.
hide account name via @media query CSS
refactor: use threshold variable
set narrow threshold to 700px
set narrow threshold to 700px
hide logo on narrow screens via @media query
remove display settings from account menu
display back-to-top arrow on narrow screens via @media query
remove narrow layout
rewrite 9c4abe89c8d3a0 to use @media queries
hide known issues icon with @media query
hide tree logo only at width < 400px
Merge branch 'fluid-layout'