GPExpansion

Update version to 0.1.5a and enhance mailbox functionality

• Introduce self-mailbox feature allowing instant creation of mailboxes on containers
• Update README.md to reflect new features and configuration options
• Allow regular GP users to create mailboxes and new mailbox-protocol for virtual, stackable mailboxes
• New setup wizard flow for creation of mailboxes
• Improve currency formatting for sign displays
• Quick fix: display renter on rented signs instead of claim owner

Full changelog: 0.1.4a...0.1.5a