Set up html-proofer as the link checker

Following configuration from
alphagov/tech-docs-template#187

Gemfile

@@ -11,3 +11,6 @@ gem 'govuk_tech_docs'
 gem 'therubyracer'
 
 gem 'middleman-gh-pages'
+
+# Include linter to check for dead internal links
+gem 'html-proofer'

Gemfile.lock

@@ -39,6 +39,8 @@ GEM
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0.6.0)
     erubis (2.7.0)
+    ethon (0.12.0)
+      ffi (>= 1.3.0)
     eventmachine (1.2.7)
     execjs (2.7.0)
     fast_blank (1.0.0)
@@ -66,6 +68,14 @@ GEM
     hamster (3.0.0)
       concurrent-ruby (~> 1.0)
     hashie (3.6.0)
+    html-proofer (3.13.0)
+      addressable (~> 2.3)
+      mercenary (~> 0.3)
+      nokogiri (~> 1.10)
+      parallel (~> 1.3)
+      rainbow (~> 3.0)
+      typhoeus (~> 1.3)
+      yell (~> 2.0)
     http_parser.rb (0.6.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
@@ -75,6 +85,7 @@ GEM
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
     memoist (0.16.0)
+    mercenary (0.3.6)
     method_source (0.9.2)
     middleman (4.3.5)
       coffee-script (~> 2.2)
@@ -130,9 +141,9 @@ GEM
       middleman-core (>= 3.2)
       rouge (~> 2.0)
     mini_portile2 (2.4.0)
-    minitest (5.12.2)
+    minitest (5.13.0)
     multi_json (1.14.1)
-    nokogiri (1.10.4)
+    nokogiri (1.10.5)
       mini_portile2 (~> 2.4.0)
     openapi3_parser (0.5.2)
       commonmarker (~> 0.17)
@@ -150,6 +161,7 @@ GEM
     rack (2.0.7)
     rack-livereload (0.3.17)
       rack
+    rainbow (3.0.0)
     rake (13.0.0)
     rb-fsevent (0.10.3)
     rb-inotify (0.10.0)
@@ -173,16 +185,20 @@ GEM
     thor (0.20.3)
     thread_safe (0.3.6)
     tilt (2.0.10)
+    typhoeus (1.3.1)
+      ethon (>= 0.9.0)
     tzinfo (1.2.5)
       thread_safe (~> 0.1)
     uglifier (3.2.0)
       execjs (>= 0.3.0, < 3)
+    yell (2.2.0)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
   govuk_tech_docs
+  html-proofer
   middleman-gh-pages
   therubyracer
   tzinfo-data

config.rb

@@ -1,7 +1,23 @@
 require 'govuk_tech_docs'
 
+# Check for broken links
+require 'html-proofer'
+
 GovukTechDocs.configure(self, livereload: { js_host: "localhost" })
 
+after_build do |builder|
+  begin
+    HTMLProofer.check_directory(config[:build_dir],
+      { :assume_extension => true,
+        :disable_external => true,
+        :allow_hash_href => true,
+        :url_swap => { config[:tech_docs][:host] => "" } }).run
+  rescue RuntimeError => e
+    abort e.to_s
+  end
+end
+
+
 DOCS_LOCATION_IN_GEM = Bundler.rubygems.find_name('govuk_tech_docs').first.full_gem_path + "/docs"
 
 files.watch :source, path: DOCS_LOCATION_IN_GEM