Config for site group to use for langlinks.

This allows the site group to show as language links to be
configurable, instead of always using the wiki's own group.
This is important for use cases like Wikimedia Commons.

(Note: this has unit tests, but I havn't tested it in the UI)

bug: 52788
Change-Id: I4e71671c8b617ba74f6d3b2b5734350e99d855c4
Merge "Config for site group to use for langlinks."