XWIKI-6882 : Improvements on user changing password

This should solve most of the issues of XWIKI-6882 but the one with the authentication cache (but I would try to do it soon). 

Note that I've had to create a new page 'XWiki.ChangePassword' to do the password change instead of the former password.vm. 
Indeed, to check the current password, we have to go through the oldcore, and so I made a groovy script veryfying this. But as groovy needs PR and that I didn't want to let all users pages have PR, I thought it was safer to do it in a different page.