DEV: let reply_by_email, visit_link_to_respond email strings be modified by plugins (#27133)

* DEV: allow reply_by_email, visit_link_to_respond strings to be modified by plugins

* DEV: separate visit_link_to_respond and reply_by_email modifiers out

lib/email/message_builder.rb

@@ -4,14 +4,13 @@
 # for the body and subject
 module Email
   class MessageBuilder
-    attr_reader :template_args
+    attr_reader :template_args, :reply_by_email_key
 
     ALLOW_REPLY_BY_EMAIL_HEADER = "X-Discourse-Allow-Reply-By-Email"
 
     def initialize(to, opts = nil)
       @to = to
       @opts = opts || {}
-
       @template_args = {
         site_name: SiteSetting.title,
         email_prefix: SiteSetting.email_prefix.presence || SiteSetting.title,
@@ -19,12 +18,23 @@ module Email
         user_preferences_url: "#{Discourse.base_url}/my/preferences",
         hostname: Discourse.current_hostname,
       }.merge!(@opts)
-
       if @template_args[:url].present?
         @template_args[:header_instructions] ||= I18n.t(
           "user_notifications.header_instructions",
           @template_args,
         )
+        @visit_link_to_respond_key =
+          DiscoursePluginRegistry.apply_modifier(
+            :message_builder_visit_link_to_respond,
+            "user_notifications.visit_link_to_respond",
+            @opts,
+          )
+        @reply_by_email_key =
+          DiscoursePluginRegistry.apply_modifier(
+            :message_builder_reply_by_email,
+            "user_notifications.reply_by_email",
+            @opts,
+          )
 
         if @opts[:include_respond_instructions] == false
           @template_args[:respond_instructions] = ""
@@ -40,9 +50,9 @@ module Email
             string =
               (
                 if allow_reply_by_email?
-                  +"user_notifications.reply_by_email"
+                  +@reply_by_email_key
                 else
-                  +"user_notifications.visit_link_to_respond"
+                  +@visit_link_to_respond_key
                 end
               )
             string << "_pm" if @opts[:private_reply]