feat: add getQuestion instance method to form field schema

[karrui]

Problem

Whilst migrating utils/question.js, realise that exported function can be inlined into the base field schema instead.

This PR adds a new instance method getQuestion() to retrieve the question for display in form responses.
Tests are also added for the new instance method.

Note that base field schemas still do not have tests written for them. This should be done in a separate PR. Issue #102 is created to track this issue.

Related to #8

Solution

Features:

• Replace utils/question with FormField.methods.getQuestion to retrieve questions of form fields.
  • utils/question is deleted

Bug fixes:

• Add validation in table fields to ensure that there must be at least 1 column in the column array.

Tests

• Submit a form with all fields.
  • The responses sent to the admin/form-filler email for email forms should have the correct questions for all fields (table fields should be ${title} (${column titles separated by comma})
  • The responses sent to the results tab for storage mode forms should also display the correct questions.

[karrui]

Tests are failing as the mocked form_fields objects in the tests are not truly a mongoose document :(. Will do a deeper check to see if it is guaranteed to be a form field schema

Tests fixed.

[liangyuanruo]

the need for this is telling me that we're missing a factory method to create form field objects - we should eventually put one on FieldSchema.

[karrui]

Yes, since FormFields are not registered as a model, we can't create them directly. Wouldn't creating a method still require the model to be registered?

[liangyuanruo]

lgtm