Add option to add someone to a path when manually entering a donation

[ineffyble]

Suggestion: http://nationbuilder.com/add_option_to_add_someone_to_a_path_when_manually_entering_a_donation

Should be a case of adding a new form select, populating with the paths list (pulled via AJAX), then either when the donation form is submitted, or when a separate "Add to path" button is clicked, posting that.

GET data: Get list of paths via AJAX - see https://github.com/ineffyble/nationtweaker/blob/master/nationtweaker.js#L202 for details

POST data: post to /signups/NBID/paths. Only form data is
path_journey[path_id]:PATHID