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.
Cleanup code, and refactor tests that still use Rugged. After this, there should
be no Rugged code that access the instance's repositories on non-test
environments. There is still some rugged code for other tasks like the
repository import task, but since it doesn't access any repository storage path
it can stay.
Without this parameter, every load of a Wiki page will load all the Wiki pages
in the repository for the sidebar. This is a significant performance penalty
that can significant slow the display of all Wiki pages.
Relates to #40101
