Programmatically check perk preconditions

[Phil25]

Current way of checking perk preconditions is by specifying which classes or weapons it's available for through the config. This greatly limits perk variety because it's impossible to determine whether a a perk is accessible in many different cases, for example:

• when the player does not have a weapon using ammo (Infinite/Extra Ammo is available to Demoknights),
• when the player has or has no buildings built (there cannot exist a perk which does something to them),
• when the player is confined within too small an area (there cannot exist a perk which enlarges the player),
• and many more.

Furthermore, editing available classes or weapons through a config can be removed. This means server operators will no longer be able to change them without updating perk's code, use case of which is minuscule, if existent at all.