[DS-1188] Collection view doesn't show content by default

https://jira.duraspace.org/browse/DS-1188

This is a pull request for the XMLUI version. The configuration on wether or not to use recent submissions on a collection home page was added to the discovery configuration spring file. I choose this route to keep all configuration together instead of spreading it out.
On all pages where recent submissions are shown a link "show more" is displayed, so even if this option isn't enabled people will be able to browse through recent submission easily.

Ps: This solution also required discovery to be enabled, but since it is the default for DSpace 4.0, I don't think this should be an issue (no use in doing double work).