Feature/fs 1342 add filestack field tag

[staturecrane]

This PR adds the Filestack field tag for to the FormHelpers. It can be used similarly to the v2 implementation, with a few differences:

• New field injects a standard button, as well as an invisible text field to hold the value of the returned file (this is necessary because we do not have form support in the v3 Picker)
• Instead of passing a general onchange callback, pass in a callback symbol in the options hash which defines which the function to be called. This function is expected to take a single argument, which represents the data returned from the Picker promise, and must either be the name of the function ('function', not 'function(data)') or a function wrapped in parentheses ('(function(data){console.log(data)})') that can be executed.

<%= f.filestack_field :filestack_url, 'button content', callback: 'callbackFunction' %>

[coveralls]

Coverage increased (+6.3%) to 96.25% when pulling 0462bfc on feature/FS-1342-add-filestack-field-tag into 92be0c0 on develop.