Standardize and materialize 500, 404, and 403 pages

In this PR:

• Removed bootstrap CSS
• Added inline Material Design Lite CSS
• Added a single CSS file for ease of editing, which can be minified and copy+pasted into the inline styles
• Updated CSS (changed old red to new red, removed extraneous CSS)
• Added very basic tab-selection function
• Added sad bucky
• Moved alternate links into tabs (uw-system links are shown by default and uw-madison links can be seen by clicking the uw-madison tab)
• Added npm to serve locally for dev purposes
• Updated readme

Note: Previously, bootstrap CSS was being brought in via CDN and other styles were added via <style> tags. Using the latter approach for MDL looks pretty gory in the HTML. Can I use a CDN for that like we were doing for bootstrap or would that be considered a dependency?

Screenshots

Notify: @timothy-vertein @levett @michael-adas @andrew-petro