Include the URLs of Zinnia under "zinnia" application and instance namespace by default to prevent backward compatibility
Revert "Include the URLs of Zinnia under "zinnia" application and instance namespace by default to prevent backward compatibility"

- Does not work as expected, the namespace is not registered
- The namespace must be registered at the root URLs level

This reverts commit 538cdf07681f2191041e7b637a862bf14b2b29cc.
Remove "zinnia_" prefix on all url names
Use "zinnia" namespace for reversing the URLs in templates, and remove "zinnia_" prefix
Use "zinnia" namespace for reversing the URLs in code, and remove "zinnia_" prefix
Register zinnia URLs under "zinnia" namespace in the demo
Make the test pass
Updating the documentation with the "zinnia" namespace
Merge branch 'feature/app-namespace' into develop, close #282