fix status bar to bottom of the screen

[derhuerst]

Fixes #3677.

• Visually polishes the status bar and uses position: fixed to show it at the bottom.
• Makes the bar a bit smaller, so it uses up less precious screen space.
• <Snackbar> has a dark grey border now, so users are able to differentiate it on dark backgrounds.

[coveralls]

Changes Unknown when pulling 60a8aab on jr-fixed-status into ** on master**.

[jacogr]

Spacing before/between blocks, or combine it, e.g.

enode {
  & > * {
    ...
  }

  & > :last-child {
    ...
  }
}

[jacogr]

Minor blank-lines and/or block issue, but fine otherwise.