Add the authored_date and authored_name of last version to WikiPage

When we render a wiki page we show the authored_date y authored_name from the last version of the wiki page. That requires an additional call to Gitaly to retrieve the last version.

When we browse the pages page, we show a bunch of pages and each one of them triggers an additional request to get the last version.

That's all the use we do of the last version right now. If we include these fields inside the wiki pages when we retrieve them, we can save a lot of RPC calls.

/cc @DouweM @jramsay