refactor: migrate checkbox validator to typescript

[tshuli]

Problem

Part of #7

Solution

Improvements

• Migrates checkbox validation to Typescript

Tests

• Create two required checkbox fields. Hide one of them behind logic. Check that normal submission using frontend succeeds for both visible and hidden field.
• Check that following validation works when submitting programatically
• Answer array cannot be empty for required field
• Answer array cannot have less than the specified minimum number of options
• Answer array cannot have more than the specified maximum number of options
• Answer array cannot have duplicate responses from fieldoption
• Answer array cannot have answers which are not in the fieldoption
• If 'others' is enabled, the others response cannot be empty (i.e. answer in answerArray cannot be 'Others: ')
• Answer array can have two identical 'Others' responses as long as one of them is a valid fieldoption, apart from enabling others

Unit test coverage

Checkbox validation
    Required or optional
      ✓ should disallow empty submission if checkbox is required
      ✓ should allow empty submission if checkbox is optional
    Validation of field options
      ✓ should disallow responses submitted for hidden fields 
      ✓ should allow a valid option to be selected
      ✓ should allow multiple valid options to be selected
      ✓ should disallow answers not in fieldOptions
      ✓ should disallow duplicate answers 
      ✓ should allow self-configured others options in field options
      ✓ should allow Others option to be submitted if field is configured for Others 
      ✓ should disallow Others option to be submitted if field is not configured for Others
      ✓ should disallow multiple Others option to be submitted if field is configured for Others
      ✓ should disallow Others option to be submitted with blank answer if field is configured for Others
      ✓ should allow submission without Others option even if field is configured for Others 
    Selection limits
      ✓ should disallow more answers than customMax if selection limits are configured 
      ✓ should disallow fewer answers than customMin if selection limits are configured 
      ✓ should allow more answers than customMax if selection limits are not configured
      ✓ should allow fewer answers than customMin if selection limits are not configured
      ✓ should disallow more answers than customMax, and fewer answers than customMin, if selection limits are configured 

[liangyuanruo]

Missing tests

Existing test coverage appears sufficient.

[mantariksh]

could you double check that the implementation of othersRadioButton validation is correct?

also, we disallow saving of forms with duplicate checkbox field options on the frontend, and we invalidate responses with duplicates at the point of submission. should we add the corresponding model validation for checkbox fields as well?

[mantariksh]

if I'm right about the implementation of othersRadioButton, we definitely need test coverage on that edge case. it's scary that none of our many layers of tests caught it.

[tshuli]

@mantariksh comments addressed, for re-review

[tshuli]

@mantariksh comments addressed, for re-review

[mantariksh]

approving first as the code looks good. great job on thinking about all the edge cases and covering them in the tests!