Add backend support for local_only on status create

Based off ThibG implementation on glitch #502

app/models/status.rb

@@ -130,6 +130,10 @@ class Status < ApplicationRecord
     attributes['local'] || uri.nil?
   end
 
+  def local_only?
+    local_only
+  end
+
   def reblog?
     !reblog_of_id.nil?
   end
@@ -222,6 +226,8 @@ class Status < ApplicationRecord
 
   around_create Mastodon::Snowflake::Callbacks
 
+  before_create :set_locality
+
   before_validation :prepare_contents, if: :local?
   before_validation :set_reblog
   before_validation :set_visibility
@@ -445,6 +451,10 @@ class Status < ApplicationRecord
     self.local = account.local?
   end
 
+  def set_locality
+    self.local_only = reblog.local_only if reblog?
+  end
+
   def update_statistics
     return unless public_visibility? || unlisted_visibility?
     ActivityTracker.increment('activity:statuses:local')