Date columns are only stored as date, not datetime

[raphaelm]

No description provided.

[raphaelm]

requery/requery#894
However, that is unfortunately vastly backwards-incompatible.

[raphaelm]

Okay, so here's the problem:

• With Date, requery creates a TEXT column on SQLite where it stores the datetime in text, which is fine for our Android apps. However, on PostgreSQL it creates a DATE column which loses the time component.
• With java.sql.Timestamp, starts storing Unix timestamps on SQLite, and who-knows-what on PostgreSQl.
• The "clean" solution would probably be to convert all of libpretixsync to java.sql.Timestamp, although I really do not like storing Unix timestamps.
• This would be very backwards-incompatible and we'd need to migrate the internal databases of all pretixSCAN and pretixPOS installations, which I would like to avoid for now.
• Therefore, we'll make a special-case solution for the QueuedCheckIn model and explicitly store the datetime as text on all databases, which is absolutely good enough for our usecase.
• However, that means that all other datetimes in libpretixsync stay unsafe on PostgreSQL for now!