CSCD-9214: Allowed specifying file extensions for pages and blocks. Also:
- Files count in number of assets created/skipped and in total progress
- Added an extra option for overwrite behavior - skipping existing so that a migration can be continued at another time in case if it was aborted for any reason (assets aren't re-created or re-edited)
- Fixed the back button in AssignFields.jsp
- More code cleanup