Fix #751 broken admin links

This is a different approach to solving the broken admin links caused by #750, i.e. it's a competitor for #752  

It partly reverts the offending PR by not prepending list paths with `/keystone/`, thus making sure that any other code depending on "clean" list paths still function.  Instead it uses an `external` property which is checked to render the admin links correctly: just as before if `external` is `false` (which it is by default) and taking the raw `list.path` value if `true`.

I think this is ATM the more appropriate approach since it makes sure everything works as before, but simply adds some functionality, i.e. if there are any bugs in the code they should arise only when external links are declared.

I am so sorry this fix is necessary :panda_face: