Reorganize series page list and series summary card class names

- Use namespaced class names to prevent accidental name collisions in
  the future
- Clean up whatever the hell I was doing with the flex config on the
  outer series list. I have no clue why it's set up as a wrapping row,
  or what the border radius is for. It seems to look the same if I just
  make it a page-list that gets its gap overridden.
- Remove page.series wrapper around the summary card render which was
  accidentally left in after 8eb343132c6b57f692f87a72a5aca1bc9c563d1b.

layouts/series/summary.html

@@ -13,7 +13,7 @@
         </div>
     {{ end }}
 
-    <div class="page-list series-inner">
+    <div class="page-list page-list--series-inner">
         {{ range .Data.Pages.Reverse }}
             {{ .Render "summary" }}
         {{ end }}