Revitalized and simplified a lot of things.

- added design folder with drafts of the new look
- moved text files to models package
- rearranged models to multiple files
- attempts to unify models' interface and make their internals more sane
  (I am still unsatisfied with the code A LOT, but I have no time now
  to make it better.)
- dropped a lot of bullshit stuff and premature optimizations
- unified slugs between URLs and filenames
- moved TODO from text file to GitHub issues
- moved caching from MongoDB to plain filesystem cache