Fix several instances of players not receiving rewards

[enteryournamehere]

• Fix mailbox not loading when the character that sent one of the mails has been deleted
• Completing an achievement when your inventory is full will now send the reward to your mailbox and show an animation
• Attempting to pick up several items at a time while your inventory is almost full will make both items disappear on the client, even though they don't all fit. If you have less than 20 mails in your mailbox, the extra items will now be sent there
• Implement mail notification request handler (shows the number of unread mails after joining a world)