update BE to better handle account page

[janethuangg]

proposing 2 changes

• Card documents will have a field "isTemplateCard" that defaults to false; during the deck creation flow, it is set to true for the deck owner's card
• the GET /user endpoint will return the following:

  { email, 
  name,
  cards: [
    {
         cardData,
         deckId,
         deckName,
         isTemplateCard,
    },
    ...
  ]
  }