- Re-enable orphan warnings that were manually disabled in many modules
- Remove non-Haskell files that were just sitting in the source tree
- Remove modules that were not called from anywhere
- Remove unused dependencies

This is not exhaustive by any means. In particular, some more weeding out
can be achieved by looking at individual functions and even branches of
functions. Weeder can help with that.

This is a first stepping stone in avoiding bugs by having `UserId` and
`ContextId` being proper types, and not synonyms.

We also refactor things to have `AuthenticatedUser` carry the `UserId`.

This is safer than type Limit = Int