Otherwise, if doc wasn't visited yet, codimd returns some large HTML
page.

It forces programmers to think about errors we are logging and reporting
to the frontend, because they need to contain no sensitive data.

WARNING: This sets the last mod time as UTC. Issue filed here for
explanation:
https://github.com/jgm/zip-archive/issues/67

It's faster now, makes less joins, doesn't need VALUES.

This brings in an important bug fix so that the output of `-Txdot_json`
is deterministic.

Apparently `dot` has a long lasting bug (recently fixed) which is the
root of the non-determinism:

https://gitlab.com/graphviz/graphviz/-/issues/2510

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.

It's not enough to check that the list is empty, we also need to check
that we have some data to begin with, i.e. we need to skip calling
`toFstDate` or `toLstDate` if we have a list of empty strings.