refactor webapp model code that checks for submission completion

r? @mstriemer

**Refactoring**

- is_complete (returned a bool + list of reasons) => details_complete (bool) + details_report (list of reasons)
- is_fully_complete (returned a bool + list of reasons) => is_fully_complete (bool) + completion_report (object of bools)
- completion_report => details_complete + content_ratings_complete + payments_complete

**Additions**

- incomplete_msgs: list of reason messages on why app is incomplete (includes all details + content_ratings + payments), used in app status API
- tests

**Fixes:**

- fix toggling payment type to not set status to PENDING if app incomplete