add ability to unset mailprefix, as well as override prefix per Mailer instance

[takuy]

This will allow you to set $conf['mailprefix'] to !!not set!!, to avoid prepending a subject prefix entirely. This method of config is borrowed from other features where null/empty string is not the same as being not set (ie manager, remoteuser and some plugin configs).

This also also introduces a function to set an email/class instance-specific prefix - using the Mailer->setSubjectPrefix() function, overriding the site-wide mailprefix and/or the page title.

[splitbrain]

first question: why? what's the motivation here?

[takuy]

first question: why? what's the motivation here?

The mailprefix just isn't that useful for emails for me or my audience, so my main goal is just to turn it off, but also have flexibility in setting it per email if needed.

Since there's only 1 event for all of email, there's no appropriate place to do this as a plugin without copy and pasting the entire email class. This change doesn't break any existing behavior.

For context, using a combination of plugins and my own code, I built out a system in Dokuwiki that allows our users to template and stage email content, where they can then send it out to our mailing list platform. There's no real prefix that needs to be there, it's all used internally for a non-wiki audience.