v3.9

• Added a new LOCK_FILE_NAME which lets you change post office's lock file name. Thanks @Mogost!
• Fixes a bug where email_queued signal is not sent in certain cases. Thanks @diesieben07!
• Fixes an issue where attachment admin page would not render with large number of emails. Thanks @petrprikryl!
• Fixes a crash when email instances are made with context, but without a template. Thanks @pacahon!
• Other miscellaneous fixes and house keeping tasks by @Mogost!