rename app/file status terms on mkt only (bug 895560)

- Copy ```amo.STATUS_CHOICES``` to ```amo.MKT_STATUS_CHOICES```.
- Create a ```MKT_STATUS_FILE_CHOICES```` that changes 'Disabled by Mozilla' to 'Obsolete'.
- Change all instances of ```amo.STATUS_CHOICES``` to ```amo.MKT_STATUS_CHOICES``` in ```mkt``` codebase.
- Change ```versions.Version.status``` to detect ```settings.MARKETPLACE``` to display ```amo.MKT_STATUS_FILE_CHOICES```