Make .Site.Params.image default on other pages

If a page does not have .Params.image defined, it will use
.Site.Params.image in the jumbotron.

This means all pages will hae jumbotron images.
Signed-off-by: Ethan Madison <ethan@ethanmad.com>

layouts/partials/jumbotron.html

 {{ $isHomePage := eq .Title .Site.Title }}
+{{ $baseUrl := .Site.BaseURL }}
 
-<div class="jumbotron" style="background-image: url({{ if $isHomePage }}{{ with .Site.Params.image }}{{ . }}{{ end }}{{ else }}{{ with .Params.image }}{{ . }}{{ end }}{{ end }})">
+<div class="jumbotron" style="background-image: url({{ if isset .Params "image" }}{{ .Params.image }}{{ else }}{{ with .Site.Params.image }}{{ if $isHomePage }}{{ . }}{{ else }}{{ $baseUrl }}/{{ . }}{{ end }}{{ end }}{{ end }});">
     <div class="container">
         <h1>{{ if $isHomePage }}{{ .Site.Title }}{{ else }}{{ .Title }}{{ end }}</h1>
         <p>{{ if $isHomePage }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ else }}{{ with .Params.description }}{{ . }}{{ end }}{{ end }}</p>