Add skip_content param to wiki find page call

In gitlab#336792 (closed) we're changing the way to render wiki pages and do it in an async way. For that, we need to avoid loading unnecessary data.

The only method we have to retrieve the page is the find_page method. This method always loads the content and we need a way to flag not to do it.

In this MR we're changing the RPC related to finding the wiki page to accept this new param and avoid loading the content when it's true.

Related: gitlab#336792 (closed)