Fixed #1348 corrected page_language_url on non CMS managed urls

The language chooser wasn't translating the urls on non managed CMS
views. simple_language_changer was deprecated since now that's the
default behavior.

Added the base 2.4 release notes.
Added IN DEVELOPMENT for the release notes and explained the deprecation warning for simple_language_changer
Fixed page_language_url for subpages in apphooks

There some leakage in the tests, running them in different order changes
the outcome.
Testcase for #1366. Created a testcase and a crafted plugin to demonstrate the issue
Added testcase for page objects for #1366. This issue doesn't apply to page-defined placeholders as codepath differs
Passing the context instance as object rather than dictionary
Passing the current_app parameter to the rendering methods
Setting the current_app on the context based on the page reverse_id and the app_name attribute of the CMSApp instance
Adding applications to the urlconf using application and instance namespaces
Correcting a bug in the AppRegexUrlResolver in order to support nested resolvers
Improve docs on PlaceholderField
Clarify request doubts in PlaceholderField docs
Fix zoom null values before schemamigration
Added more meaningful message upon migration. Added query filter suggested by @BertrandBordage
remove all the i18n garbage not needed anymore.
Support for nested page reversing
Renumbered last migrations for clarity
Extending the CMSPageMiddleware class to set the current_app attribute on the request as well using a lazy proxy
Adding an additional app hook with its view and urls
Implemented test cases for app hook urls reversing using instance and application namespaces