Option to change the email subject prefix

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

lib/email/message_builder.rb

@@ -21,7 +21,7 @@ module Email
       @to = to
       @opts = opts || {}
 
-      @template_args = {site_name: SiteSetting.title,
+      @template_args = {site_name: SiteSetting.email_prefix.presence || SiteSetting.title,
                         base_url: Discourse.base_url,
                         user_preferences_url: "#{Discourse.base_url}/user_preferences" }.merge!(@opts)