fix bbpress installation link for wordpress multisite 3.1

[sbrajesh]

Using current trunk #4077 with wordpress multisite 3.1, when you try to setup new bbpress forum using the Forums setup, the install new link/reinstall links are broken.
this is because site_url is used instead of network_admin_url.
Here is a patch attached. It should work in wordpress single user too, as the network_admin_url faklls back to admin_url for non multisite install.

[boonebgorges]

(In [4078]) Swaps site_url() calls on forum setup Dashboard page to network_admin_url(). Fixes #3093. Props sbrajesh

[johnjamesjacoby]

Moving closed ticket out of Awaiting Review.