Organizer-level plugins (#5305)

* Add version notes to the docs

* Adapt signal handling

* Add UI

* Add API

* API and tests

* Fix registry

* Update doc/development/api/plugins.rst

Co-authored-by: Felix Rindt <felix@rindt.me>

* Fix failing tests

* Apply suggestions from code review

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Update src/pretix/control/templates/pretixcontrol/organizers/plugin_events.html

Co-authored-by: luelista <weller@rami.io>

* Update src/pretix/control/templates/pretixcontrol/organizers/plugins.html

Co-authored-by: luelista <weller@rami.io>

* Update src/pretix/control/templates/pretixcontrol/organizers/plugins.html

Co-authored-by: luelista <weller@rami.io>

* Update src/pretix/control/navigation.py

Co-authored-by: luelista <weller@rami.io>

* Update src/pretix/control/urls.py

Co-authored-by: luelista <weller@rami.io>

* Apply suggestion from @wiffbi

* REbase migration

* Fix review note

* Fix test cases

* Remove plugin from all events if disabled on org level

* Update doc/development/api/plugins.rst

* Unify registries

* Rebase migration

---------

Co-authored-by: Felix Rindt <felix@rindt.me>
Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: luelista <weller@rami.io>

src/tests/testdummy/apps.py

@@ -19,10 +19,10 @@
 # You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
 # <https://www.gnu.org/licenses/>.
 #
-from django.apps import AppConfig
+from pretix.base.plugins import PluginConfig
 
 
-class TestDummyApp(AppConfig):
+class TestDummyApp(PluginConfig):
     name = 'tests.testdummy'
     verbose_name = '.testdummy'