(bug 38201) Support multiple site link groups.

This enables support for having multiple groups of wikis that
site links can point to.

This change is a first draft. It is incomplete in that:
* the JavaScript UI needs to support this too.
* internationalization needs to be added for at least the default group.
* uniqueness constraints may not yet work correctly.
* ...and probably more.

Change-Id: Icb65789fa5f04c836cb39d89b13f762ac68b7ca5
Merge "(bug 38201) Support multiple site link groups."