Allow styling broadcast messages

Adds the ability to style links in broadcast messages and use line
breaks to format messages.

changelogs/unreleased/nicolasdular-allow-broadcast-message-styling.yml

+---
+title: Allow styling broadcast messages
+merge_request: 21522
+author:
+type: added

lib/banzai/filter/broadcast_message_sanitization_filter.rb

+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    # Sanitize HTML produced by Markdown.
+    #
+    # Extends Banzai::Filter::BaseSanitizationFilter with specific rules.
+    class BroadcastMessageSanitizationFilter < Banzai::Filter::BaseSanitizationFilter
+      def customize_whitelist(whitelist)
+        whitelist[:elements].push('br')
+
+        whitelist[:attributes]['a'].push('class', 'style')
+
+        whitelist[:css] = { properties: %w(color border background padding margin text-decoration) }
+
+        whitelist
+      end
+    end
+  end
+end

lib/banzai/pipeline/broadcast_message_pipeline.rb

@@ -6,7 +6,7 @@ module Banzai
       def self.filters
         @filters ||= FilterArray[
           Filter::MarkdownFilter,
-          Filter::SanitizationFilter,
+          Filter::BroadcastMessageSanitizationFilter,
 
           Filter::EmojiFilter,
           Filter::ColorFilter,

spec/lib/banzai/pipeline/broadcast_message_pipeline_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Banzai::Pipeline::BroadcastMessagePipeline do
+  before do
+    stub_commonmark_sourcepos_disabled
+  end
+
+  subject { described_class.to_html(exp, project: spy) }
+
+  context "allows `a` elements" do
+    let(:exp) { "<a>Link</a>" }
+
+    it { is_expected.to eq("<p>#{exp}</p>") }
+  end
+
+  context "allows `br` elements" do
+    let(:exp) { "<a>Link</a>" }
+
+    it { is_expected.to eq("<p>#{exp}</p>") }
+  end
+end