Fixes #13: Add validation handler to 3 post requests

[haianhng31]

Resolves #13

• Create a function validate_fields:

  • Inputs: required_fields, request_data
  • Output: boolean, error_message

• In each of the post requests:

  • Check whether it has all the required fields using the function above
  • If it is, then add that new request into data object
  • Return the json object back to frontend accordingly

• Add 6 test cases for 3 post requests:

  • One test case for successful creation of a new entry
  • One test case for unsuccessful creation because of missing fields