Extensions pages

Add or update the following Extensions page: Home, Extensions Team Overview, phpBB Official Extensions, Extensions Development Team Overview, Installing, Extensions Rules and Guidelines, Recognised Extension Developers.
Ah okay, done: https://github.com/battye/phpbb-website/commit/203aa646f00f4ca3948458db788e308a1f6af433. 
I'll work through your notes (and make a new pull request?)... unit tests up next :smile: