[BACKEND] Group password

[hn275]

Handling optional password for groups.

• [x] Change Mongoose schema to include an optional password field.
• [x] Password needs to be encrypted. (bcrypt)
• [x] Differentiate private and public group.
• User validation, using httpOnly cookie so no client side JavsScript can access it:
  • [x] API route needs to parse Cookie from headers, nookie can help.
  • [x] Redirecting user if Cookie expired/user not authenticated.
  • [x] Destroy Cookie when user log out (is there a logout feature?).