improve image handling for edge cases Fixes #11

layouts/partials/page-header.html

-{{ $featured_image := $.Param "featured_image" }}
+{{ $featured_image := .Params.featured_image }}
 {{ if $featured_image }}
-  <header class="cover bg-center" style="background-image: url('{{ $featured_image | absURL }}');">
+  {{/* Trimming the slash and adding absURL make sure the image works no matter where our site lives */}}
+  {{ $featured_image := (trim $featured_image "/") | absURL }}
+  <header class="cover bg-center" style="background-image: url('{{ $featured_image }}');">
     <div class="pb3-m pb6-l bg-black-60">
       {{ partial "site-navigation.html" . }}
       <div class="tc-l pv6 ph3 ph4-ns">

layouts/partials/site-header.html

-{{ $featured_image := $.Param "featured_image" }}
+{{ $featured_image := .Params.featured_image | default .Site.Params.featured_image }}
 {{ if $featured_image }}
-  <header class="cover bg-center" style="background-image: url('{{ $featured_image | absURL }}');">
+  {{/* Trimming the slash and adding absURL make sure the image works no matter where our site lives */}}
+  {{ $featured_image := (trim $featured_image "/") | absURL }}
+  <header class="cover bg-center" style="background-image: url('{{ $featured_image }}');">
     <div class="bg-black-60">
       {{ partial "site-navigation.html" .}}
       <div class="tc-l pv4 pv6-l ph3 ph4-ns">

layouts/partials/summary-with-image.html

+{{ $featured_image := .Params.featured_image }}
 <article class="bb b--black-10">
   <a class="db pv4 ph3 ph0-l no-underline dark-gray dim" href="{{ .URL }}">
     <div class="flex flex-column flex-row-ns">
       {{ if .Params.featured_image }}
+          {{/* Trimming the slash and adding absURL make sure the image works no matter where our site lives */}}
+        {{ $featured_image := (trim $featured_image "/") | absURL }}
         <div class="pr3-ns mb4 mb0-ns w-100 w-40-ns">
-          <img src="{{ .Params.featured_image }}" class="db" alt="image from {{ .Title }}">
+          <img src="{{ $featured_image }}" class="db" alt="image from {{ .Title }}">
         </div>
       {{ end }}
       <div class="w-100{{ if .Params.featured_image }} w-60-ns pl3-ns{{ end }}">