[CLEAN] mail, tools: factorize decode email tools

mail.mail and mail.thread have methods to decode headers. This commit
move them in mail tools. It is indeed simplier to have all email tools
in the same file. Moreover they have been renamed to avoid confusion
with standard library methods: decode and decode_header already exist.

 * decode in mail_message is now decode_smtp_header
 * decode_header in mail_thread is now decode_message_header

A method to data from references is also moved into tools. A regex was
already defined in tools. Now the method using it is already put in tools.
Mailgateway will now simply call this method.

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