Merge branch '6022-ee-extract-slash_commands-in-ce' into 'master'

Backport EE SlashCommand Refactor

See merge request gitlab-org/gitlab-ce!19177

lib/gitlab/slash_commands/command.rb

 module Gitlab
   module SlashCommands
     class Command < BaseCommand
-      COMMANDS = [
+      def self.commands
+        [
           Gitlab::SlashCommands::IssueShow,
           Gitlab::SlashCommands::IssueNew,
           Gitlab::SlashCommands::IssueSearch,
           Gitlab::SlashCommands::IssueMove,
           Gitlab::SlashCommands::Deploy
-      ].freeze
+        ]
+      end
 
       def execute
         command, match = match_command
@@ -37,7 +39,7 @@ module Gitlab
       private
 
       def available_commands
-        COMMANDS.select do |klass|
+        self.class.commands.keep_if do |klass|
           klass.available?(project)
         end
       end