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".

This enables sending a chat message to Slack or Mattermost
  upon a successful, failed, or canceled deployment

Partially addresses #47424.

[10.6] Prevent notes on confidential issues from being sent to chat

See merge request gitlab/gitlabhq!2366
# Conflicts:
#	app/helpers/services_helper.rb

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

An upcoming update to rubocop-gitlab-security added additional
violations.

Use the EnforcedStyleForMultiline: no_comma option.

Signed-off-by: Rémy Coutable <remy@rymai.me>

The notify_only_default_branch property is using boolean_accessor
this means we need to check it using a question methods.
Also add specs for disabling this option.

Right now, it once  a chat notifacation service has been enabled, there
is no way to limit the branches which trigger a notification. Instead of
allowing the user to specify the list, I opted to let the user check the
box if they'd only want to be notified of the default branch.

Tags are uneffected by this change.

[ci skip]