layouts: update partials to _partials

Hugo v0.146.0 introduced a
[new template system](https://gohugo.io/templates/new-templatesystem-overview/)
that changed the structure of the `layouts` directory.

Commit https://github.com/jpanther/congo/commit/99d5cf8b28ce82377588310799420721943a3e65
updated this theme to support the new `layouts` directory structure but
missed a few updates to `templates.Exists` calls. This patch fixes those
calls.

layouts/_partials/head.html

@@ -103,7 +103,7 @@
     {{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink ($.Site.Title | emojify) | safeHTML }}
   {{ end -}}
   {{/* Icons */}}
-  {{ if templates.Exists "partials/favicons.html" }}
+  {{ if templates.Exists "_partials/favicons.html" }}
     {{ partialCached "favicons.html" .Site }}
   {{ else }}
     <link rel="apple-touch-icon" sizes="180x180" href="{{ "apple-touch-icon.png" | relURL }}" />
@@ -146,7 +146,7 @@
   {{/* Analytics */}}
   {{ partial "analytics.html" . }}
   {{/* Extend head - eg. for custom analytics scripts, etc. */}}
-  {{ if templates.Exists "partials/extend-head.html" }}
+  {{ if templates.Exists "_partials/extend-head.html" }}
     {{ partial "extend-head.html" . }}
   {{ end }}
 </head>