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

config/locales/server.en.yml

@@ -819,7 +819,7 @@ en:
     email_in: "Allow users to post new topics via email"
     email_in_address: "The email address the users can post new topics to. None means users can't post globally."
     email_in_min_trust: "The minimum trust level an users needs to have to be allowed to post new topics via email"
-    email_in_category: "The category new emails are posted into"
+    email_prefix: "The [label] used in the subject of emails. It will default to 'title' if not set"
 
     minimum_topics_similar: "How many topics need to exist in the database before similar topics are presented."
 

config/site_settings.yml

@@ -269,6 +269,7 @@ email:
     default: 3
     enum: 'MinTrustToCreateTopicSetting'
   email_in_category: -1
+  email_prefix: ''
 
 files:
   max_image_size_kb:

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)
 

spec/components/email/message_builder_spec.rb

@@ -145,10 +145,15 @@ describe Email::MessageBuilder do
   context "template_args" do
     let(:template_args) { builder.template_args }
 
-    it "has the site name" do
+    it "has the site name as the site title when `SiteSetting.email_prefix` is not set" do
       expect(template_args[:site_name]).to eq(SiteSetting.title)
     end
 
+    it "has the site name as SiteSetting.email_prefix when it is set" do
+      SiteSetting.email_prefix = 'some email prefix'
+      expect(template_args[:site_name]).to eq(SiteSetting.email_prefix)
+    end
+
     it "has the base url" do
       expect(template_args[:base_url]).to eq(Discourse.base_url)
     end