Allow option to clear certain properties during XML import

For better supportability, we should prompt users in the XML Import page a checkbox option to clear out values from the backup such as:

• Outbound/Inbound Mail Servers

It seems too common where users restore a backup, and find their test/dev JIRA instances picking up mail messages from their production mailbox. It can be quite a headache to reconcile all the issues if this goes unnoticed for a while.