XAR Extensions installed as "install on farm" are not added when new wikis are created later.

To repeat:

• Install an extension using "install on farm" option
• create a new subwiki
• try to access the extension on the new subwiki.

Because "install on farm" actually means "install in every subwiki" it's behaving logically, but since it's labeled "install on farm", it is expected that it should be available in subwikis even which do not yet exist.