Extract sync instance method

Lots of the stuff going on inside the Subject.sync method is acting on
an instance and not doing things at the class level. This commit extracts
a Subject#sync instance method to make things more object oriented and
to allow us to start extracting the logic out of this method. This
should hopefully make things easier to understand what's going on.

app/models/subject.rb

@@ -44,9 +44,7 @@ def sync_involved_users
     involved_user_ids.each { |user_id| SyncNotificationsWorker.perform_in(1.minutes, user_id) }
   end
 
-  def self.sync(remote_subject)
-    subject = Subject.find_or_create_by(url: remote_subject['url'])
-
+  def sync(remote_subject)
     # webhook payloads don't always have 'repository' info
     if remote_subject['repository']
       full_name = remote_subject['repository']['full_name']
@@ -56,31 +54,36 @@ def self.sync(remote_subject)
       full_name = extract_full_name(remote_subject['url'])
     end
 
-    comment_count = remote_subject['comments'] || remote_subject.fetch('commit', {})['comment_count']
-    comment_count = subject.comment_count if comment_count.nil?
+    updated_comment_count = remote_subject['comments'] || remote_subject.fetch('commit', {})['comment_count']
+    updated_comment_count = comment_count if updated_comment_count.nil?
 
-    subject.update({
+    update({
       repository_full_name: full_name,
       github_id: remote_subject['id'],
       state: remote_subject['merged_at'].present? ? 'merged' : remote_subject['state'],
       author: remote_subject.fetch('user', {})['login'],
       html_url: remote_subject['html_url'],
       created_at: remote_subject['created_at'] || Time.current,
       updated_at: remote_subject['updated_at'] || Time.current,
-      comment_count: comment_count,
+      comment_count: updated_comment_count,
       assignees: ":#{Array(remote_subject['assignees'].try(:map) {|a| a['login'] }).join(':')}:",
       locked: remote_subject['locked'],
       sha: remote_subject.fetch('head', {})['sha'],
       body: remote_subject['body'].try(:gsub, "\u0000", ''),
       draft: remote_subject['draft']
     })
 
-    return unless subject.persisted?
+    return unless persisted?
 
-    subject.update_labels(remote_subject['labels']) if remote_subject['labels'].present?
-    subject.update_comments if Octobox.include_comments? && (subject.has_comments? || subject.pull_request?)
-    subject.update_status
-    subject.sync_involved_users if (subject.saved_changes.keys & subject.notifiable_fields).any?
+    update_labels(remote_subject['labels']) if remote_subject['labels'].present?
+    update_comments if Octobox.include_comments? && (has_comments? || pull_request?)
+    update_status
+    sync_involved_users if (saved_changes.keys & notifiable_fields).any?
+  end
+
+  def self.sync(remote_subject)
+    subject = Subject.find_or_create_by(url: remote_subject['url'])
+    subject.sync(remote_subject)
   end
 
   def self.sync_status(sha, repository_full_name)
@@ -243,7 +246,7 @@ def github_client
     end
   end
 
-  def self.extract_full_name(url)
+  def extract_full_name(url)
     url.match(/\/repos\/([\w.-]+\/[\w.-]+)\//)[1]
   end