Finalize base template/styling; add _variables.

Add:
    static/sass/main.sass
        -Complete `template/layout.html` positioning rules.
        -Add some styling to the footer.

    static/sass/_variables.sass
        -Add `SASS` partial to contain all project-wide variables.

    templates/index.html
        -Add project-wide layout template (should've been packaged into the
        previous commit).