rewrite and integrate get user endpoint

[kdduan]

As title. Integrated the GET user endpoint to use MongoDB and rewrote the endpoint logic to decrease FE complexity.

Sample Response:

[
  {
    isOwned: false,
    deckName: 'new',
    cardData: {
      _id: '618c1528d02027aa33890a2d',
      deckId: '618c1528d02027aa33890a2b',
      name: 'Janet Huang',
      city: 'NYC',
      tagline: '~5 word tagline about yourself',
      summary: 'ROLE (# YOE), working hours & time zone',
      sectionContent0: 'What do you excel at?',
      sectionContent1: 'What do you struggle with?',
      sectionContent2: 'How do you want people to contact you?',
      sliderValue: 50,
      __v: 0
    }
  }
]