[ticket/12167] Allow users to change style via style parameter by default

This change will enable users that don't have the a_styles permission to
preview styles using the style parameter. Using this, they will be able to
use extensions that are or incorporate stylechangers.

PHPBB3-12167
Merge pull request #2008 from marc1706/ticket/12167

[ticket/12167] Allow users to change style via style parameter by default