Reorganize gdsc app

[tibbben]

I like the separation of filters.html from the index template.html. It has inspired me to consider reorganizing the templates and the python application more ...

templates:

• consider breaking the index and the details pages into smaller pieces that are imported
  • the card for datasets in the index
  • the cards for attributes and other pieces of the details

gdsc.py:

• consider moving all structured specifications (filter specs, bibliography specs) to another file
• consider moving all support functions to another file
• better document functions (see features/more-filters branch)