Danger-bot: More meaningful maximum PR size message (#3517)

Dangerfile

@@ -5,11 +5,21 @@ EXCLUSIONS = ['Gemfile.lock', '.json', 'spec/fixtures/', '.txt', 'spec/support/v
 # takes form {"some/file.rb"=>{:insertions=>4, :deletions=>1}}
 changed_files = git.diff.stats[:files]
 
+excluded_changed_files = changed_files.select { |key| EXCLUSIONS.any? { |exclusion| key.include?(exclusion) } }
 filtered_changed_files = changed_files.reject { |key| EXCLUSIONS.any? { |exclusion| key.include?(exclusion) } }
 lines_of_code = filtered_changed_files.sum { |_file, changes| (changes[:insertions] + changes[:deletions]) }
 
 if lines_of_code > MAX_PR_SIZE
-  warn("PR is exceeds `#{MAX_PR_SIZE}` LoC. Consider breaking up into multiple smaller ones.")
+  msg = <<~HTML
+    You changed `#{lines_of_code}` LoC. This exceeds our desired maximum of `#{MAX_PR_SIZE}`. Big PRs are difficult to review and often become stale. Consider breaking this PR up into smaller ones.
+
+    **Calculation Summary**
+    - #{filtered_changed_files.collect { |key, val| "#{key} (+#{val[:insertions]}/-#{val[:deletions]} )" }.join("\n- ")}
+
+    **Exclusions**
+    - #{excluded_changed_files.collect { |key, val| "#{key} (+#{val[:insertions]}/-#{val[:deletions]} )" }.join("\n- ")}
+  HTML
+  warn(msg)
 end
 
 # Warn when a PR includes a simultaneous DB migration and application code changes