(bug 39397) Only show rollback links if they work

To only show rollback links if they work I had to patch Linker
to have a function (Linker::rollbackData) which can verify
that the editor isn't the only editor of the page. Furthermore
it is checked that the user name or the text of the revision
we might rollback to isn't deleted. Due to the fact that I've
altered the already existing method which showed how many edits
a rollback will revert for that, this wont affect the performance.

Change-Id: I5d1adec993370c39ae8c5c712edd919d456441c6