Reconsider structure of notes table

[kyoshizaki]

As the function of note increases, the status column becomes complicated, so create a category column and describe the state of the note more clearly

• add category column to note table
  • default value for category column is "private"
• change default value for status column to "draft"
• note.master_id => note.original_note_id

[kyoshizaki]

Done!