feat: use textarea for providing checkbox options

[jia1]

Problem

We may expect users to have many Checkbox options and adding them one-by-one to FormSG is troublesome. As such, we want Checkbox options to follow Dropdown options so that Checkbox options can be edited by a textarea with each option in each line in the textarea.

Closes #743

Solution

Only frontend changes. Edit the conditional rendering (i.e. ng-if) such that Dropdown and Checkbox show the same textarea for Options and Radio button will show its usual one-by-one options.

In the controller, edit the if clauses so that we now transform fieldOptions into fieldOptionsFromText when rendering Options and we check for fieldOptionsFromText instead of manualOptions when saving fieldOptions.

Before & After Screenshots

BEFORE:

AFTER:

Note: I have not checked for test breakages yet.

[karrui]

lgtm