Make URLs consistent, more readable and extensible in the future

[ahirner]

I propose to stick to the following format:

- /type_of_page/<project>/<filename>/
- /show/muni_bonds/EP..... [for example]

By that we get rid of most query string parameters. This is already done in the xirr-specific branch.
Furthermore, in future we should reference documents alternatively by a short hash or from a database ID like so:
- /show/a74a3AhP/

For API endpoints, we stick to prefixing them with /api but basically mirror the project/filename subfolders.