Reduce amount of property definitions in ChatNotificationService

Before this commit we redefined properties for supported events on each
new instance of ChatNotificationService during "runtime".

This commit defines the properties only once during "compile time".

app/models/project_services/chat_notification_service.rb

@@ -5,17 +5,25 @@
 class ChatNotificationService < Service
   include ChatMessage
+  SUPPORTED_EVENTS = %w[
+    push issue confidential_issue merge_request note confidential_note
+    tag_push pipeline wiki_page deployment
+  ].freeze
+
+  EVENT_CHANNEL = proc { |event| "#{event}_channel" }
+
   default_value_for :category, 'chat'
   prop_accessor :webhook, :username, :channel
+
+  # Custom serialized properties initialization
+  prop_accessor(*SUPPORTED_EVENTS.map { |event| EVENT_CHANNEL[event] })
+
   boolean_accessor :notify_only_broken_pipelines, :notify_only_default_branch
   validates :webhook, presence: true, public_url: true, if: :activated?
   def initialize_properties
-    # Custom serialized properties initialization
-    self.supported_events.each { |event| self.class.prop_accessor(event_channel_name(event)) }
-
     if properties.nil?
       self.properties = {}
       self.notify_only_broken_pipelines = true
@@ -32,8 +40,7 @@ class ChatNotificationService < Service
   end
   def self.supported_events
-    %w[push issue confidential_issue merge_request note confidential_note tag_push
-       pipeline wiki_page deployment]
+    SUPPORTED_EVENTS
   end
   def fields
@@ -139,7 +146,7 @@ class ChatNotificationService < Service
   end
   def event_channel_name(event)
-    "#{event}_channel"
+    EVENT_CHANNEL[event]
   end
   def project_name