feat: add support for page-specific headers (navbars)

Both site header and footer now use dynamic block system.

E.g. for custom header at `layouts/partials/components/headers/simple.html:

```
---
header:
  navbar:
    block: simple
---
```

Also, Params.footer.widget changed to Params.footer.block

modules/wowchemy/layouts/_default/baseof.html

@@ -37,7 +37,13 @@
   {{ end -}}
 
   <div class="page-header header--fixed">
-    {{ partial "navbar" . }}
+  {{/* Load header block */}}
+  {{ $block_name := .Params.header.navbar.block | default site.Params.header.navbar.block | default "navbar" }}
+  {{ $block_path := printf "components/headers/%s.html" $block_name }}
+  {{ if not (templates.Exists (printf "partials/%s" $block_path)) }}
+    {{ errorf "Check your `header` config in `params.yaml` and/or front matter. Cannot find header block at `layouts/partials/%s`" $block_path }}
+  {{ end }}
+  {{ partial $block_path . }}
   </div>
 
   <div class="page-body">

modules/wowchemy/layouts/partials/site_footer.html

@@ -2,11 +2,11 @@
 
   {{ partial "functions/get_hook" (dict "hook" "footer-start" "context" .) }}
 
-  {{/* Load footer widget */}}
-  {{ $footer_section_name := site.Params.footer.widget | default "minimal" }}
+  {{/* Load footer block */}}
+  {{ $footer_section_name := site.Params.footer.block | default "minimal" }}
   {{ $footer_section_path := printf "components/footers/%s.html" $footer_section_name }}
   {{ if not (templates.Exists (printf "partials/%s" $footer_section_path)) }}
-    {{ errorf "Check your `params.yaml`. Cannot find footer widget at `layouts/partials/%s`" $footer_section_path }}
+    {{ errorf "Check your `params.yaml`. Cannot find footer block at `layouts/partials/%s`" $footer_section_path }}
   {{ end }}
   {{ partial $footer_section_path . }}