Fix already initialized constant constant warning

lib/gitlab/email/handler.rb

@@ -3,8 +3,6 @@
 module Gitlab
   module Email
     module Handler
-      HANDLER_ACTION_BASE_REGEX = /(?<project_slug>.+)-(?<project_id>\d+)-(?<incoming_email_token>.+)/.freeze
-
       def self.handlers
         @handlers ||= load_handlers
       end

lib/gitlab/email/handler/base_handler.rb

@@ -6,6 +6,8 @@ module Gitlab
       class BaseHandler
         attr_reader :mail, :mail_key
 
+        HANDLER_ACTION_BASE_REGEX ||= /(?<project_slug>.+)-(?<project_id>\d+)/.freeze
+
         def initialize(mail, mail_key)
           @mail = mail
           @mail_key = mail_key

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::HANDLER_ACTION_BASE_REGEX}-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::HANDLER_ACTION_BASE_REGEX}-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)