Add adv-search form, skeleton-pages.

Add:
    templates/index.html
        -Add compartmental `div` structure for the advanced-search form.

    static/sass/index.sass
        -Add layout style rules to the advanced-search form.

    app.py, templates/(about, developers).html
        -Add `Flask` routes and skeleton files for pages to be developed later.

Ref:
    static/js/index.js
        -Substitute some verbose `JavaScript` calls with their `jQuery`
        equivalents.