Make dependencies on dc.contributor.* configurable

In order to support future evolutions of the dspace metadata schema, DCAT identified a need for developers to be able to phase out metadata fields in favor of new ones. In the current codebase, we see challenges for a number of fields because of hard coded references to these fields. As a result, "phasing out" such fields would require a great amount of changes on different locations of the code.

We think those hard coded dependencies need to be identified and made configurable as a first step.

A main priority here are those fields that are represented as qualified dublin core, e.g. specified by both a field name and a qualifier. The prime example here is dc.contributor.*, such as dc.contributor.author.

This is why we are proposing, as the scope for this particular JIRA ticket, to identify all hard coded occurrences of dc.contributor.* fields in the codebase, and to make them configurable in a next step.

Examples where such hard coded references exist include

Item Mapper search:
https://github.com/DSpace/DSpace/blob/master/dspace-xmlui/src/main/java/org/dspace/app/xmlui/aspect/administrative/mapper/SearchItemForm.java#L117

RSS feeds:
https://github.com/DSpace/DSpace/blob/master/dspace-api/src/main/java/org/dspace/app/util/SyndicationFeed.java#L84

PDF Packager:
https://github.com/DSpace/DSpace/blob/master/dspace-api/src/main/java/org/dspace/content/packager/PDFPackager.java#L341

What remains to be decided is where the configuration should be place. It could be as simple as adding the following to dspace.cfg but maybe there is a need for more subtle configuration:

#Main metadatafield where the repository is storing information about the author of an item.
#The data model assumes that there is at least one value per item for this field.
author = dc.contributor.author

The general context for this JIRA request is the ongoing work from DCAT to refine requirements to align DSpace with current metadata standards.
https://wiki.duraspace.org/display/DSPACE/Proposal+to+Update+DC+Registry+and+Add+DCTERMS+Registry

We believe that a first step in this direction would be the identification and removal of any hard-coded dependencies on specific metadata fields in the DSpace codebase. Once configurable, any migrations to different fields should in theory be easier as changes will be more centralized.