Revert "Merge branch '847-combine-polling-queries-to-one-polling-query' into 'release-3.0'"

See merge request !1235

This reverts merge request !1229

Resolve "Combine polling queries to one polling query"

See merge request !1229

Resolve "404 seems to be broken again"

See merge request !1228

Merge branch '832-make-graphql-graphene-mutations-actually-return-error-messages' into 'release-3.0'

Resolve "Make GraphQL/Graphene mutations actually return error messages"

See merge request !1210

Merge branch '832-make-graphql-graphene-mutations-actually-return-error-messages' into 'release-3.0'

Resolve "Make GraphQL/Graphene mutations actually return error messages"

See merge request !1210

Resolve "Urls in invitation email contains the /django/ prefix"

See merge request !1217

# Conflicts:
#   CHANGELOG.rst

Merge branch '843-celery-backend_cleanup-task-fails-due-to-foreign-key-constraint' into 'release-3.0'

Resolve "Celery backend_cleanup task fails due to foreign key constraint"

See merge request !1234

Merge branch 'release-3.0' into '843-celery-backend_cleanup-task-fails-due-to-foreign-key-constraint'

# Conflicts:
#   CHANGELOG.rst

Resolve "Links in object column of data check table is broken"

See merge request !1233

Resolve "Fix displaying of messages in Vue frontend"

See merge request !1232

Merge branch '812-dynamic-routes-are-not-removed-in-frontend-immediately-when-they-are-removed-in-the-backend' into 'release-3.0'

Resolve "Dynamic routes are not removed in frontend (immediately) when they are removed in the backend"

See merge request !1227

Merge branch 'release-3.0' into '812-dynamic-routes-are-not-removed-in-frontend-immediately-when-they-are-removed-in-the-backend'

# Conflicts:
#   CHANGELOG.rst

Merge branch '833-progress-page-for-celery-tasks-doesn-t-show-error-messages-if-task-aborts-with-an-exception' into 'release-3.0'

Resolve "Progress page for Celery tasks doesn't show error messages if task aborts with an exception"

See merge request !1226

Merge branch 'release-3.0' into '833-progress-page-for-celery-tasks-doesn-t-show-error-messages-if-task-aborts-with-an-exception'

# Conflicts:
#   CHANGELOG.rst

Resolve "Remove vue_dummy"

See merge request !1224

Merge branch '838-offline-fallback-page-for-django-legacy-pages-is-misleading-in-some-situations' into 'release-3.0'

Resolve "Offline fallback page for django legacy pages is misleading in some situations"

See merge request !1225