Add "User Cleanup" component

As some users wanted to have an option to remove user profiles if they remove the corresponding users from the LDAP, they also think it is useful in general and thus asked be to publish it to contrib.

See also: https://forum.xwiki.org/t/request-for-two-small-contrib-extensions/9382/6

Description from there:

What the extension does is defining a scheduler job that looks for pages with an XWiki.LDAPProfileClass object and then checks if the corresponding user in the LDAP still exists.
If this is not the case, it removes the corresponding user profile page and optionally moves all “document creator”, “document author” and “comment author” entries matching that user to a (configurable) placeholder account.