Added Profile_prefs class for profile preferences

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.

db/core.php

@@ -59,6 +59,7 @@ $classes = array('Schema_version',
                  'Invitation',
                  'Message',
                  // 'Notice_inbox',
+                 'Profile_prefs',
                  'Profile_tag',
                  'Profile_list',
                  'Profile_tag_subscription',