Added deadline field to ProjectPlan model

PR adds deadline field to ProjectPlan model. Updates ProjectPlan serializer and Ember models. Includes migrations. The deadline is available to see in the ProjectPlan admin. The deadline field is only used in the project create flow and the backoffice (confirmed with Bart).

Initially the deadline was added to Project but, after discussing with Bart, the field was moved to ProjectPlan (Campaign). As a result, the file changes show some fiddling with fields on the Project model (including a migration).