Add ability to rob bank and fix bugs with gun/ammo

Ability to rob the bank has now been implemented.

Some bugs around this have been fixed:

• User can only rob bank/shop if at least 10 bullets are owned
• Ammo is displayed correctly in the inventory page
• Once purchased, a gun can no longer be purchased in pawn shop (previously a blank menu item was selected by default, which would purchase a second gun)
• User receives the correct number of bullets. Previously, when purchasing 5 bullets, only 1 would be added to inventory