Send all the fields in the Need API request on create and update

At present, when a user tries to blank an optional field in Maslow, it is not included in the request to the Need API and so not updated. This branch changes the Need model so that all fields are included in a create or update request to the Need API as long as they exist, regardless of whether they are blank or not.

This also includes a fix where leading newline characters are included in the two textarea fields. We now strip these out in the `Need.as_json` method.