Avoid the use of deprecated MariaDB "zero dates" globally. If they're present
as attribute defaults somewhere, they will be replaced with NULL implicitly.
The existing "zero dates" in MariaDB storage will be left intact and this
should not present any issues.
The "timestamp" type in table definitions now corresponds to DATETIME in
MariaDB with "DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP", which
should be close enough to the original behaviour for compatibility purposes.
It is now the recommended type for "modified" attributes, because of the
update trigger on MariaDB. But there is no such trigger implemented on
PostgreSQL as of this moment.
Profile_prefs aims to consolidate all the profile preferences into a
single table. Otherwise we end up with a bajillion *_prefs classes, like
User_urlshortener_prefs, or new fields in existing User/Profile classes,
like 'urlshorteningservice', 'homepage', 'phone_number', 'pet_name' etc.
Eventually we should migrate as many user-settable preferences as we can
into this system.
The data in Profile_prefs is organized by:
* profile_id Identify the current Profile.
* namespace Which plugin/section the preference is for.
* topic Preference name (like 'homepage')
* data Preference data (like 'https://gnu.org/')
The names 'topic' and 'data' are because 'key' and 'value' may be rather
ambigous when dealing with our DB_DataObject classes etc.
