Remove XWikiUser disabled property and introduce email_checked property

We introduced in XWiki 11.6RC1 a new property on XWikiUsers objects called disabled to allow to disable or enable users. This property was however redundant with the previously existing active property, which was until then only used to check if users validated their email account, when the linked registration option was activated. 

In this release, we remove the disabled property and rely entirely on the active property to know if a user account is enabled or not. We introduced a new email_checked property to verify if a user validated his/her email address if the option was activated.