In my previous commit I had changed their implementation to use the
FromJSON/ToJSON istances, making sure those were backward compatible,
but apparently that wasn't the case.

After Fabien's changes, this needed amendment.

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

This commit moves away from generic-derive JSON istances for `NodeType`,
as we know that the frontend will be relying on this in the GraphQL API,
and we don't want them to silently break the frontend during
refactoring.

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.