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.
The change in !11728 would cause an arbitrary project to be chosen to test
system hooks, and it's likely that the project would not have any commits or
relevant commits to test the hook. This would prevent admins from verifying
that the hook fired. Instead of trying to create a representative hook
dynamically, just send static data to guarantee the hook will actually be
tested.
Closes#37067
