🔥(backend) remove CollaborationService and can-edit endpoint

The CollaborationService was doing nothing since we started the
migration to yhub, all the code using it is now removed. Also the
`can-edit` endpoint and all the safeguard mechanism relying on the
presence of other users connected to the websocket will not be used
anymore, it will be possible to replace all of this with yhub, so all
this code is also removed.

src/helm/env.d/dev/values.impress.yaml.gotmpl

@@ -15,9 +15,6 @@ image:
 backend:
   replicas: 1
   envVars:
-    COLLABORATION_SERVER_SECRET: my-secret
-    COLLABORATION_API_URL: https://docs.127.0.0.1.nip.io/collaboration/api/
-    COLLABORATION_WS_NOT_CONNECTED_READ_ONLY: False
     CONVERSION_UPLOAD_ENABLED: True
     DJANGO_CSRF_TRUSTED_ORIGINS: https://docs.127.0.0.1.nip.io
     DJANGO_CONFIGURATION: Feature

src/helm/env.d/feature/values.impress.yaml.gotmpl

@@ -16,10 +16,7 @@ image:
 backend:
   replicas: 1
   envVars:
-    COLLABORATION_SERVER_SECRET: my-secret
     CONVERSION_UPLOAD_ENABLED: True
-    COLLABORATION_API_URL: https://{{ .Values.feature }}-docs.{{ .Values.domain }}/collaboration/api/
-    COLLABORATION_WS_NOT_CONNECTED_READ_ONLY: True
     DJANGO_CSRF_TRUSTED_ORIGINS: https://{{ .Values.feature }}-docs.{{ .Values.domain }}
     DJANGO_CONFIGURATION: Feature
     DJANGO_ALLOWED_HOSTS: {{ .Values.feature }}-docs.{{ .Values.domain }}