FIX: Clear drafts only when post is created by real user (#15720)

This commits adds a new advance_draft to PostCreator that controls if
the draft sequence will be advanced or not. If the draft sequence is
advanced then the old drafts will be cleared. This used to happen for
posts created by plugins or through the API and cleared user drafts
by mistake.

app/controllers/posts_controller.rb

@@ -182,6 +182,7 @@ class PostsController < ApplicationController
   def create
     @manager_params = create_params
     @manager_params[:first_post_checks] = !is_api?
+    @manager_params[:advance_draft] = !is_api?
 
     manager = NewPostManager.new(current_user, @manager_params)