Support mapping of user entries to existing accounts

When importing authors, mentions and any reference to users, it would be needed to be able to provide a mapping file to convert the users to an already existing account in XWiki.

The file would be stored on the server and contain fields like:

currentvalue1=newvalue1
currentvalue2=newvalue2
currentvalue3=newvalue3
currentvalue4=newvalue4

In the case of a Confluence import this would be run before or after the user conversion already being done.

For example currently a user mention or page author is stored as XWiki.ludovic@xwiki_com
If the file contains

ludovic@xwiki_com=ldubost

Then the mention should be

XWiki.ldubost

This task is also related to CONFLUENCE-73 as authors and mentions should be xwiki:XWiki.ldubost in the end.