Allow valid lists to contain invalid sites

Add a method to obtain only targets valid for delivery to content
objects and a warning when viewing lists with invalid targets. This
prevents config changes from suddenly making revisions inaccessible.

Also hide the site field when there is only one site, even if
wgAllowGlobalMessaging is true.

Change-Id: If45a0ef56c2f92fd8a7debceea0e39c4dd532ba8