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.