Make client robust against unknown sites.

If the client wiki received links from the repo that contain
references to wiki sites that are not known on the client wiki
(due to differences in the sites table), this should trigger a
warning, but not a fatal error.

LIGHTLY TESTED. The change handling codepath needs another
look (ClientChangeHandler).

Change-Id: Ie21744510af795022b4c585d4e668a7662181b50
Merge "Make client robust against unknown sites."