(bug 43997) UI for linking articles to Wikidata items from the client

This introduces an UI which allows linking articles with wikibase items
directly on the clients in case there aren't any langlinks yet.
This can basically handle two cases:

1. The page the user wants to link the current page with already has an
   item: In this case we show all pages linked with the item to the user
   and ask for confirmation. If the users confirms we link the current
   page with the item. In case the current page already has an item
   (which only links the current page) the page link from that item
   will be removed in favour of the one the user wants to link with.

2. The page the user wants to link the current article with hasn't yet
   got an item: In this case we create a new item or if the current
   page already has an item we add the user selected page to that item.

This required moving a lot of messages from repo to lib as thos will
be used on the client as well. Furthermore I had to modify RepoApi
slightly.

Please note that this feature is only available for users who are
logged in on both the client and the repo. If that's not the case
and error is shown.

Change-Id: I087e0960e3177adddd0529677a6b1d0de3667ded
Merge "(bug 43997) UI for linking articles to Wikidata items from the client"