Migrate XWikiPreferences captchas properties prettyName

Migrate the prettyName of the following properties in the XWikiPreferences class since they are not helpful at all (they do not state which actions the captcha apply for). This would allow us to integrate those properties easily using the current AdminDisplayFieldsSheet fromt the administration application. Properties concerned :

• registration_anonymous (current prettyName : "Anonymous")
• registration_registered (current prettyName : "Registered")
• edit_anonymous (current prettyName : "Anonymous")
• edit_registered (current prettyName : "Registered")
• comment_anonymous (current prettyName : "Anonymous")
• comment_registered (current prettyName : "Registered")

A solution to do this is to write a new migrator in com.xpn.xwiki.store.migration.hibernate;