Use time-literals

[egli]

Currently the source-date of an item is parsed and serialized as a java.util.Date. When reading it from the db with edn/read-string it will be converted back to java.util.Date. The rest of the code base uses java.time and local-date.

It would be nice if we could move all to java.time. For that we need to enhance edn to store and read the types of java.time. There is a library which does that namely time-literals by Henry Widd.

One (major) downside of this is that we would have to migrate all existing edn strings in the database.