Repeater permission fix

This is a fix that was discussed [at the forums back in 2013](https://processwire.com/talk/topic/2291-homepage-slide-images-repeater-cropmodule-not-showing-if-guest/?p=37102), though the context was probably different. The issue is that users without permissions to edit admin pages can't seem to crop images within Repeaters, and the fix is that instead of checking from the RepeaterPage if it's editable by current user, we'll ask it from the forPage, which (to me, at least) seems like the correct behaviour anyway.

@apeisa, could you take a look at this and say if you see any issues with this?

It kind of feels like it would be the job of ProcessWire itself to forward editable() checks for a RepeaterPage to the page containing it, though perhaps I'm wrong -- that would essentially make it impossible to check if the RepeaterPage itself is editable, which might be required in some situations. Either way, I'm a bit confused about this whole thing, so would appreciate second opinion :)

This also fixes a Strict Standards error ("only variables should be passed by reference") related to earlier Repeater fixes.