DS-1945 RequestItem to have Helpdesk be contacted instead of author

This adds an override for the RequestItem feature for embargoed works, but instead of the Submitter of the work getting the request, a "helpdesk" email address, would instead get the request.

https://jira.duraspace.org/browse/DS-1945

This builds off of @ottenhoff's Pull Request, but changes it from a hard-coded implementation, to instead allow for Spring Dependency Injection to determine which RequestItem Author implementation gets called.
https://github.com/DSpace/DSpace/pull/505

To enable this feature: 

config/dspace.cfg
```
# Helpdesk E-mail
mail.helpdesk = ${mail.admin}
# Should all Request Copy emails go to the helpdesk instead of the item submitter?
request.item.helpdesk.override = true
```

config/spring/api/requestitem.xml, comment out MetadataStrategy, and uncomment HelpdeskStrategy
```xml
<bean class="org.dspace.app.requestitem.RequestItemHelpdeskStrategy"
        id="org.dspace.app.requestitem.RequestItemAuthorExtractor"></bean>
```

For local testing, I found that using something like mailcatcher (fake smtp port 1025) and catches all outgoing emails from DSpace is handy.