Annotations were generated with pyannotate:
https://github.com/dropbox/pyannotate
We are running in strict mode, which is awesome!
This affects a large part of the code base, which might be an issue for
people running a fork of Tutor. Nonetheless, the behavior should not be
affected. If anything, this process has helped find and resolve a few
type-related bugs. Thus, this is not considered as a breaking change.
Configuration loading was overly complex. Here, we simplify it
drastically with reasonable defaults. Hacky additional variables are
unncessary now that we use custom jinja2 filters.
