Make 'checkboxes' input type work in custom form editor and display

Summary:
Add 'checkboxes' input mapping in FormModel
Handle options array properly in the form editor

Fixes T839

Test Plan:
Add a checkboxes field in the form editor.
Add options, save the field.
Check it shows correctly in preview.
Go to posts create, select your form and check it renders

Reviewers: aMoniker, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T839

Differential Revision: https://phabricator.ushahidi.com/D392