XWIKI-11183: Display the "Create a new space" link for users with the right to create a space

There is one small issue that comes from the fact that you are now using a Velocity variable defined outside the code of the wiki macro: someone can overwrite the value of ``$hasCreateSpace`` right before calling the ``spaces`` wiki macro.

    #set ($hasCreateSpace = true)

    {{spaces/}}

But I guess it's fine because you won't be able to do much with the create space link if you don't have the necessary rights. Generally though, I think the Velocity variables defined in ``xwikivars.vm`` should be used in templates, where they cannot be overwritten.