Merge: Extend batch export to DMS for multi-volume works (#173)

Adds functionality to export a serial publication with references to all volumes in the anchor files of each process.

**Configuration:**
* Make sure the metadata type `MetsPointerURL` is available for `Volume`:
``` xml
<MetadataType>
    <Name>MetsPointerURL</Name>
    <language name="de">METS-Verweis-URL</language>
    <language name="en">METS pointer URL</language>
</MetadataType>
<!-- ... -->
<DocStrctType>
    <Name>Volume</Name>
    <!-- ... -->
    <metadata num="*">MetsPointerURL</metadata>
</DocStrctType>
```
**Usage:**
* Add all processes belonging to the same serial publication into a batch
* Click on *Export batch with serial publication into DMS*
* You can follow the export progressing on the *Long-running tasks* screen

**Prerequisites:**
* The development needed to be based on #166 *Perform DMS exports as long running tasks*