Add thedaywefightback.org functionality. (Disabled by default)

See https://thedaywefightback.org/ for details.
See `_config.yml.example` for sample config, or add:

    thedaywefightback:
        enable: true
        testing: false

to your `_config.yml` file.  (Setting `testing` to true will
show the banner even if it's not Feb 11, which is useful for
a local `jekyll serve` to make sure it renders properly.)
Fix typos, speeling mistakes, and docs. No functional changes.