Starboard Mega-Issue #158

Making a mega-issue for all the things wrong with starboard.

• re-order DB interactions per PR reorder db interactions and add types to starboard.py #123
• add starboard msg-deletion handler per PR reorder db interactions and add types to starboard.py #123
• remove BlacklistedMessageException in favour of a StarboardMessagePair object per PR reorder db interactions and add types to starboard.py #123
• add static types to starboard per PR reorder db interactions and add types to starboard.py #123
• fix starboard post rendering when the user has left the server (top_role is not defined)
• add backup emoji for when/if :starhaj: is removed from the server (nitro moment)
• write comprehensive documentation & wiki-page for everything starboard does
• re-consider 'only one image/embed' policy now that discord condenses images