add new incorporation fields to form and sponsor model

[jessiebelle]

addressing this issue

adding the following nullable fields to the sponsor model:

• Country of incorporation
• State of incorporation

new sponsor application form includes the new fields from the sponsor model

• refactor of the new_sponsor_application_form.html to also accommodate editing sponsor information
• updated tests to reflect that the correct form is used

[ewdurbin]

My only feedback is to group the new form fields a bit further away from the mailing address bits as it will likely confuse humans and form-fillers like password managers.

[ewdurbin]

I updated the structure/order of the application form a bit with 4c32364

@jessiebelle could you update the edit form to match?