Update 'Installer' system requirements

[Groruk]

Update the 'installer' system requirement page to include checks for PHP extensions like GMP or bcmath

Expected Behavior

System requirement check should check for all dependencies needed to run the SourceBans++ webpanel.

Current Behavior

Checking for basic stuff like database version, PHP version, etc but not for specific extensions like GMP, bcmath, etc.

Possible Solution

Include all dependencies.

Context

Helps to solve environment specific issues like #441 caused by missing extensions / requirements. Also allows SourceBans++ to use for example GMP for steamid calculation instead of SQL querys which will reduce database load and increase performance.