[Easy] Implement notifications for Peerplays objects

[nathanielhourt]

Bug Description
In db_notify.cpp, there is a function, get_relevant_accounts() to get accounts related to any object type in the system. Several peerplays-specific types are not recognized by this function yet, and should be added. This allows the chain to determine which accounts are affected by a modified object and then enables front-ends to notify affected users of the event.

Modern compilers give a warning when processing this code which is helpful in determining which objects still need to be handled:

db_notify.cpp:314:15: warning: 9 enumeration values not handled in switch: 'sport_object_type', 'event_object_type', 'event_group_object_type'...
db_notify.cpp:388:15: warning: 10 enumeration values not handled in switch: 'impl_collateral_bid_object_type', 'impl_asset_dividend_data_object_type', 'impl_total_distributed_dividend_balance_object_type'...

Impacts
Describe which portion(s) of Peerplays may be impacted by this bug. Please tick at least one box.

• API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• Deployment (the deployment process after building such as Docker, Gitlab, etc.)
• P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• UX (the User Experience)
• Other (please add below)

PBSA / Developer tasks

• Evaluate / Prioritize Bug Report
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation