ertp form: move issue category field
ertp form: field rename, issue category => incident stage
ertp form: incident stage moved in ticket, added to subject
ertp form: add EMS supplier field
ZendeskTicket class now manages priority
ertp form: add priority field

this field maps to the priority field on the zendesk ticket
the "gds_zendesk" gem needs to be bumped up to be able to test this.
Merge pull request #69 from alphagov/ertp-form-improvements

ERTP form improvements