Fix already initialized constant constant warning

ee/lib/gitlab/email/handler/ee/service_desk_handler.rb

@@ -10,7 +10,7 @@ module Gitlab
         class ServiceDeskHandler < BaseHandler
           include ReplyProcessing
 
-          HANDLER_REGEX        = /\A#{::Gitlab::Email::Handler::BaseHandler::HANDLER_ACTION_BASE_REGEX}-issue-\z/.freeze
+          HANDLER_REGEX        = /\A#{HANDLER_ACTION_BASE_REGEX}-issue-\z/.freeze
           HANDLER_REGEX_LEGACY = /\A(?<project_path>[^\+]*)\z/.freeze
 
           def initialize(mail, mail_key)

lib/gitlab/email/handler/base_handler.rb

@@ -6,7 +6,7 @@ module Gitlab
       class BaseHandler
         attr_reader :mail, :mail_key
 
-        HANDLER_ACTION_BASE_REGEX = /(?<project_slug>.+)-(?<project_id>\d+)/.freeze
+        HANDLER_ACTION_BASE_REGEX ||= /(?<project_slug>.+)-(?<project_id>\d+)/.freeze
 
         def initialize(mail, mail_key)
           @mail = mail

lib/gitlab/email/handler/create_issue_handler.rb

@@ -11,7 +11,7 @@ module Gitlab
       class CreateIssueHandler < BaseHandler
         include ReplyProcessing
 
-        HANDLER_REGEX        = /\A#{Gitlab::Email::Handler::BaseHandler::HANDLER_ACTION_BASE_REGEX}-(?<incoming_email_token>.+)-issue\z/.freeze
+        HANDLER_REGEX        = /\A#{HANDLER_ACTION_BASE_REGEX}-(?<incoming_email_token>.+)-issue\z/.freeze
         HANDLER_REGEX_LEGACY = /\A(?<project_path>[^\+]*)\+(?<incoming_email_token>.*)\z/.freeze
 
         def initialize(mail, mail_key)

lib/gitlab/email/handler/create_merge_request_handler.rb

@@ -12,7 +12,7 @@ module Gitlab
       class CreateMergeRequestHandler < BaseHandler
         include ReplyProcessing
 
-        HANDLER_REGEX        = /\A#{Gitlab::Email::Handler::BaseHandler::HANDLER_ACTION_BASE_REGEX}-(?<incoming_email_token>.+)-merge-request\z/.freeze
+        HANDLER_REGEX        = /\A#{HANDLER_ACTION_BASE_REGEX}-(?<incoming_email_token>.+)-merge-request\z/.freeze
         HANDLER_REGEX_LEGACY = /\A(?<project_path>[^\+]*)\+merge-request\+(?<incoming_email_token>.*)/.freeze
 
         def initialize(mail, mail_key)