Use UTC internally

[jtojnar]

According to https://dev.mysql.com/doc/refman/8.0/en/time-zone-support.html

MySQL converts TIMESTAMP values from the current time zone to UTC for storage, and back from UTC to the current time zone for retrieval. (This does not occur for other types such as DATETIME.)

That is bad.

• [x] Check that we do not use TIMESTAMP anywhere or set MySQL timezone to UTC. The latter might not be possible, as MySQL requires importing data to support named timezones, which is not possible for some hosts.
• [ ] Verify that storing date with offset into MySQL will always store it as UTC, no matter the MySQL timezone.
• [ ] Verify that PostgreSQL includes timezone offsets when selecting datetime values with timezone.
• [ ] Check that MySQL correctly compares datetimes in db with values containing tz offset as produced by daos\mysql\statements::datetime().
• [ ] Add migrations for sqlite local → UTC when date.timezone is not UTC.
• [ ] Add news.md entry.
• [ ] Fix https://github.com/mibe/FeedWriter/pull/50

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