[Feature] make `extraVolumeMounts` available for initContainers

[saw-jan]

Include extraVolumeMounts in the initContainers as well, so that the container and initContainer both can work on the same extra volumes if provided.

Similar to env:

env:
  {{- include "openproject.env" . | nindent 12 }}

      initContainers:
        - name: wait-for-db
          {{- include "openproject.containerSecurityContext" . | indent 10 }}
          image: {{ include "openproject.image" . }}
          imagePullPolicy: {{ .Values.image.imagePullPolicy }}
          envFrom:
            {{- include "openproject.envFrom" . | nindent 12 }}
          env:
            {{- include "openproject.env" . | nindent 12 }}
          args:
            - /app/docker/prod/wait-for-db
          resources:
            {{- toYaml .Values.appInit.resources | nindent 12 }}
          volumeMounts:
            {{- include "openproject.tmpVolumeMounts" . | indent 12 }}
+           {{- include "openproject.extraVolumeMounts" . | indent 12 }}

Mainly necessary for web-deployment.yaml and worker-deployment.yaml