DS-2204 Consolidate/Clean Batch Import UI code

https://jira.duraspace.org/browse/DS-2204

There were two Batch Import UI PR's recently, that each added duplication of code to ItemImport. Since they are both in, we can clean up the ItemImport code to reuse methods and clean up the code.

Peter's XMLUI:  https://github.com/DSpace/DSpace/pull/591
Kostas' JSPUI: https://github.com/DSpace/DSpace/pull/675

Primarily the unzip(File) method and other SAF and BTE methods can be consolidated.