Add support for resources in pages.

Can now display images, videos, and audio. Not well tested yet. Not
available at the option level because I'm not sure how they should be
displayed for options. CSS will be desirable for setting image size,
etc.
Add support for tags that get passed to output.

Should make analysis easier. Pages and Options can have tags. Across
pages, and separately, across options, tag arrays should be of equal
length. Option tags get zipped in case you have some nonexclusive
questions. A use case would be: it's an ABX task and you want to know
whether the chosen response was A or B without needing to merge in a map
from option IDs to option types. Not fully tested yet.
Cleanup.

Add errors to JS.
Refactor Python.
Remove commented out code.
Rename files to match experimental focus.
Reorganize directory structure.