feat(web/panettone): Make responsive

Make the site responsive, by making all the hard :widths we were using
into :max-widths, and adding a viewport meta tag.

Change-Id: I02f054f81ff57fbd1c4603b179b2104367f03e3b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1415
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

web/panettone/src/css.lisp

@@ -138,7 +138,7 @@
     (a :color "inherit")
 
     (.content
-     :width "800px"
+     :max-width "800px"
      :margin "0 auto")
 
     (header
@@ -174,7 +174,7 @@
      :background-color ,color/failure)
 
     (.login-form
-     :width "300px"
+     :max-width "300px"
      :margin "0 auto")
 
     (.created-by-at