Add sponsor to footer

Michigan Rec Sports requires their logo on our website, so it's added to
the bottom of the footer in the `sponsor.html` partial.

The logic for the partial is somewhat complex, since only one element is
needed to activate it and the image must be displayed on both root and
non-root pages.
Signed-off-by: Ethan Madison <ethan@ethanmad.com>

layouts/partials/footer.html

@@ -6,6 +6,7 @@
 <footer class="doc-footer">
 	{{ partial "social-links.html" . }}
 	{{ with .Site.Copyright | markdownify }}<p>{{ . }}</p>{{ end }}
+	{{ partial "sponsor.html" . }}
 </footer>
 
 <script src="{{ $baseUrl }}/js/jquery.min.js"></script>

layouts/partials/sponsor.html

+{{ $isHomePage := eq .Title .Site.Title }}
+{{ $baseUrl := .Site.BaseURL }}
+
+{{ if isset .Site.Params "sponsor" }}
+<p>
+{{ with .Site.Params.sponsor.link }}
+<a href='{{ . }}' target='_blank'>
+{{ end }}
+{{ if isset .Site.Params.sponsor "logo" }}
+<img src='{{ if $isHomePage }}{{ .Site.Params.sponsor.logo }}{{ else }}{{ $baseUrl }}/{{ .Site.Params.sponsor.logo }}{{ end }}' style='width: {{ with .Site.Params.sponsor.size }}{{ . }}{{ else }}100px{{ end }};' title='{{ with .Site.Params.sponsor.text }}{{ . }}{{ else }}Sponsor logo{{ end }}' alt='{{ with .Site.Params.sponsor.text }}{{ . }}{{ else }}Sponsor logo{{ end }}'>
+{{ else }}
+{{ with .Site.Params.sponsor.text }}{{ . }}{{ end }}
+{{ end }}
+{{ with .Site.Params.sponsor.link }}
+</a>
+{{ end }}
+</p>
+{{ end }}