Add comments to main_content

Added comments to main_content.html to label existing menus and note
required menus.
Create main.cpp

Added main.cpp for the purposes of requesting the game list, requesting
the session list, starting a game session, and checking for players in
lobby.
Add commands for main.cpp

Added a list of commands for main.ccp to handle. Also, added session
checking and added main.cpp to the makefile.