Add missing appconfs for settings, table_block and sites

wagtail/contrib/settings/__init__.py

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+default_app_config = 'wagtail.contrib.settings.apps.WagtailSettingsAppConfig'

wagtail/contrib/table_block/__init__.py

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+default_app_config = 'wagtail.contrib.table_block.apps.WagtailTableBlockAppConfig'

wagtail/contrib/table_block/apps.py

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+from django.apps import AppConfig
+
+
+class WagtailTableBlockAppConfig(AppConfig):
+    name = 'wagtail.contrib.table_block'
+    label = 'wagtailtableblock'
+    verbose_name = "Wagtail table block"

wagtail/sites/__init__.py

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+default_app_config = 'wagtail.sites.apps.WagtailSitesAppConfig'

wagtail/sites/apps.py

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+from django.apps import AppConfig
+
+
+class WagtailSitesAppConfig(AppConfig):
+    name = 'wagtail.sites'
+    label = 'wagtailsites'
+    verbose_name = "Wagtail sites"