Added list_pages method to avoid loading all wiki pages content

Inside a wiki, when we show the sidebar or browse to the `pages`,
all page contents are retrieved from Gitaly and that is a waste
of resources, since no content from that pages are going to be
showed.

This MR introduces the method `ProjectWiki#list_pages`,
which uses new wiki_list_pages RPC call to retrieve
pages without content

Also in the `WikisController` we're using the method to show
pages in the sidebar and also on the `pages` page.