Option to change the subject prefix in emails

It adds a new setting 'email_prefix' to configure which [label] will be used in the subject of emails. Discourse currently uses '[title]'. The problem is that sometimes you need to set a longer title, that doesn't really work well for emails. I think this is very common since the HTML `<title>` tag is very important for SEO.

It will default to '[title]' if this setting is not used.

See: https://meta.discourse.org/t/where-to-change-the-email-subject-prefix/11989