feat: add link to front page to the per-repository report page — radicle-ci-broker

Signed-off-by: Lars Wirzenius liw@liw.fi

         let title = format!("CI runs for repository {}", alias);
         Self::head(&mut doc, &title);
         doc.push_to_body(
             Element::new(Tag::P).with_child(
                 Element::new(Tag::A)
                     .with_attribute("href", "../index.html")
                     .with_text("Front page"),
             ),
         );
         Self::h1(&mut doc, &title);
         doc.push_to_body(
             Element::new(Tag::P).with_text("Repository ID ").with_child(
                 Element::new(Tag::Code)