Address code review remarks

[alexandrepiveteau]

The following remarks were made in #259 and have still have to be addressed :

• Add an explanation of the architecture of the app, and the difference between the state and ui packages; (see Add a description of the principal packages #404)
• Use some enum of constant values for Store document field names; #423
• Rename AuthenticatedUser.firestore to AuthenticatedUser.store; #421
• Comment the class parameters of AuthenticatedUser; #422
• Comment ChessFacade.StoreMatch; #419
• Missing @return comment on Board.set; (removed during chess engine rewrite ab19fa8)
• Better document what happens when setting a null piece with Board.set; (removed during chess engine rewrite ab19fa8)
• Missing comment on Color.denormalize; (removed during chess engine rewrite ab19fa8)
• Implement and refactor the castling logic; (removed during chess engine rewrite ab19fa8)
• Avoid magic numbers in Moves.kt; (removed during chess engine rewrite ab19fa8)
• Missing comments for some parameters of PersistentGame; (see Add a comment to PersistentBoard #403)
• No debouncing of input in search method for other users; and (see Debounce user input on search #402) #420
• Missing comment for @return on `Query.limit. (done in cf6f766)

Items which have been handled must be checked.