WIP: Ticket/9245 Bot management and styles

Fixes http://tracker.phpbb.com/browse/PHPBB3-9245

Does the following:

1. When the default style is changed, change all users (normal, bots and anonymous user) using the old style to the new style.

3. Handle DST and timezones for bots and anonymous users exactly the same as styles - change a bot or anonymous user's DST/timezone if it in equal to the old default, otherwise don't change.

4. Never change normal users' timezones.