fix done/not done logic on submit next steps page (bug 948098), create fixed message on devhub indicating next steps (bug 948656)

**Notes:**

- cross out "Set Up Payments" on the checklist when it is actually done.
- when lacking Content Ratings or Payments, there will be a fixed message on top of the app's devhub page.
- ```mkt.webapps.models.Webapp.is_fully_complete()``` is tested which drives everything here

![screen shot 2013-12-10 at 5 43 30 pm](https://f.cloud.github.com/assets/674727/1720550/a9172794-6205-11e3-8bc2-ee3e0fd0be72.png)

![screen shot 2013-12-10 at 5 14 02 pm](https://f.cloud.github.com/assets/674727/1720562/146f512e-6206-11e3-891a-94ecb4e715fd.png)