This was (User, Either CorpusName, [CorpusId]) before, but the case of
UserMaster doesn't make sense with these parameters, so I rewrote the
function to accept only correct datatypes as inputs.

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

This big commit improves the jobs API in a way that now we can
completely abstract away over a JobLog.

Instead use Text.Read.readMaybe and panic with custom error message.

This turned out to be quite heavy because of module import cycles.