Update graphic assets, add to respective locale directories

[wasade]

As identified by Brittanie:

• https://github.com/biocore/microsetta-interface/blob/master/microsetta_interface/static/img/bristol_stool.jpg
• https://github.com/biocore/microsetta-interface/blob/master/microsetta_interface/static/img/hero-home-4-xl.jpeg
• https://github.com/biocore/microsetta-interface/blob/master/microsetta_interface/static/img/how-it-works.svg

[dhakim87]

Moved these to a new en_us directory. Need to add translated versions to es_mx directory. We can probably remove all the ones on the FAQ page, since it seems the faq page itself has been replaced (set to point to https://microsetta.ucsd.edu/participant-resources/faqs/ insead)