实践课程：新建和编辑页面，简介的字符数扩张到8000，须知的字符数扩张到2000

app/models/subject.rb

@@ -22,8 +22,8 @@ class Subject < ApplicationRecord
   has_many :courses, -> { where("is_delete = 0").order("courses.created_at ASC") }
 
   validates :name, length: { maximum: 60 }
-  validates :description, length: { maximum: 5000 }
-  validates :learning_notes, length: { maximum: 500 }
+  validates :description, length: { maximum: 8000 }
+  validates :learning_notes, length: { maximum: 2000 }
 
   scope :visible, lambda{where(status: 2)}
   scope :published, lambda{where(status: 1)}