Create separate GitHub Projects for XMLUI and JSPUI (post-6.x)

Related to DS-3422

Because active bug fixing is still occurring in XMLUI and JSPUI for the 6.x releases, I've delayed splitting them out into separate GitHub projects. If we were to split them out immediately, bug fixes would not be synced between the existing dspace-6_x maintenance branch and the new GiHub projects.

Once 6.x bug fixing slows (likely near 7.0 release), we should do the following:

• Split the dspace-xmlui subfolder of the dspace-6_x maintenance branch out into a "DSpace/DSpace-XMLUI" project in GItHub.
• Split the dspace-jspui subfolder of the dspace-6_x maintenance branch out into a "DSpace/DSpace-JSPUI" project in GItHub.

Splitting out subfolders into separate GitHub projects is documented at: https://help.github.com/articles/splitting-a-subfolder-out-into-a-new-repository/