Implement Reply-To header support when replying

[vermeeren]

The Reply-To header, if present, will take precedence over the From header. If the Reply-To data is undefined old behaviour is used.

Note that Horde libraries appear to internally determine reply_to property in the Horde_Imap_Client_Data_Envelope (unless Dovecot does it, but doubt). It was observed that reply_to property had the same value as from property in a mail that contained no Reply-To header. It is assumed that the Horde implementation makes sense, so just use the value as-is.

This notably fixes replying to mail from software reliant on the Reply-To header such as GitLab and GitHub notifications and many types of mail based ticketing (support) systems.

Closes #4075 Closes #7538