Allow non-GOV.UK favicon and opengraph assets

Fixes: #347

We already provide a way to non display the crown next to the service name by setting `show_govuk_logo: false` in config/tech-docs.yml. This is used by external users such as [MOJ](https://user-guide.modernisation-platform.service.justice.gov.uk).

This work extends the usage of that setting enabled custom favicon and opengraph image insted of the default Crown.
It's limited to just the `favicon.ico`, `favicon.svg` and `opengraph-image.png`, but this should suffice.

Users need to create those assets and place them in their `source/images` folder.

lib/govuk_tech_docs/meta_tags.rb

@@ -48,7 +48,11 @@ def canonical_url
     attr_reader :config, :current_page
 
     def page_image
-      "#{host}/assets/govuk/assets/images/govuk-opengraph-image.png"
+      if config[:tech_docs][:show_govuk_logo]
+        "#{host}/assets/govuk/assets/images/govuk-opengraph-image.png"
+      else 
+        "#{host}/images/opengraph-image.png"
+      end
     end
 
     def site_name

lib/source/layouts/core.erb

@@ -10,11 +10,16 @@
     <%= stylesheet_link_tag :manifest %>
 
     <link rel="canonical" href="<%= meta_tags.canonical_url %>">
-    <link rel="icon" sizes="48x48" href="/assets/govuk/assets/images/favicon.ico">
-    <link rel="icon" sizes="any" href="/assets/govuk/assets/images/favicon.svg" type="image/svg+xml">
-    <link rel="mask-icon" href="/assets/govuk/assets/images/govuk-icon-mask.svg" color="#0b0c0c">
-    <link rel="apple-touch-icon" href="/assets/govuk/assets/images/govuk-icon-180.png">
-    <link rel="manifest" href="/assets/govuk/assets/manifest.json">
+    <% if config[:tech_docs][:show_govuk_logo] %>
+      <link rel="icon" sizes="48x48" href="/assets/govuk/assets/images/favicon.ico">
+      <link rel="icon" sizes="any" href="/assets/govuk/assets/images/favicon.svg" type="image/svg+xml">
+      <link rel="mask-icon" href="/assets/govuk/assets/images/govuk-icon-mask.svg" color="#0b0c0c">
+      <link rel="apple-touch-icon" href="/assets/govuk/assets/images/govuk-icon-180.png">
+      <link rel="manifest" href="/assets/govuk/assets/manifest.json">
+    <% else %>
+      <link rel="icon" sizes="48x48" href="/mages/favicon.ico">
+      <link rel="icon" sizes="any" href="/images/favicon.svg" type="image/svg+xml">
+    <% end %>
 
     <% meta_tags.tags.each do |name, content| %>
       <%= tag :meta, name: name, content: content %>