Migrate PyMongo to Motor, model fixes and form creation route

[ks129]

• Migrated PyMongo to asynchronous Motor.
• Made small fixes in models:
  • Removed circular import of Question
  • Use cls instead of self (Pydantic stuff)
  • Fix form features validation (plain __members__.values() don't work).
• For listing views, move data to model before returning to:
  • catch broken data
  • return id instead _id
• Locked all forms listing endpoint only to admins.
• Created endpoint for creating new forms (/forms/new, POST).
• Closes Add route for creating forms #6.

[jb3]

Yet another fantastic PR, great work ks!