Add attributes to deck too

[peterpeterparker]

We need attributes on the deck entity too

Same type as for slide

data Deck = Deck
  { deckSlides :: [SlideId]
  , deckDeckname :: Deckname 
  , deckOwnerId :: UserId
  , deckAttributes :: HMS.HashMap T.Text T.Text -- NEW
  } deriving (Show, Eq)

P.S.: Use for example to reflect a background color on the all deck

<deckgo-deck style="--background: yellow; --color: red;"/>