Add:
        app.py, templates/layout.html, static/sass/main.sass
                -Remove footer's slide-up animation; the additional content
                displayed (a copyright statement) is superfluous.

Add:
        bitshift/assets.py, static/sass/404.sass
                -Add `syntax_highlight()` and style rules to perform syntax
                highlighting on 404 error messages; used in
                `templates/error404.html`.

        templates
                -Add a `Jinja` expression to select a random "404" message from
                a sequence.

Add:
        app.py, templates/error404.html, static/sass/error404.sass
                -Add Flask 404 handler and styled 404 page.