sheet.php: GAME_TITLE and navigation fixes

Fixes to the projects pages (sheet.php) correct the following:
 - Removed the 'projects' navigation item as this does not exist on the project page, and added 'features' navigation item and id as these were missing.
 - Set the game title and navigation title to GAME_TITLE
 - Add an id="game-title" to the navigation title so that this can be picked up by the email script validation.js, which fixes a bug where emails had 'undefined' in the email subject.