Fix for DS-1682 - Mirage theme header is hardcoded into XSLT.

Both fixes DS-1682 and also changes the default header text (in messages.xml) to be "DSpace Repository" instead of "DSpace/Manakin Repository"

https://jira.duraspace.org/browse/DS-1682

I believe these changes should be non-controversial, but I'll leave this PR open for a few days for comment.