[lib/dbmodels.rb, lib/ske48gplus-activity.rb] now ignore youtube

  - when video is youtube, set status to 3 (IMAGE_STATE_INVALID_URL) and never try to download.
  - add new value 'youtube' as image_type
[lib/debmodels.rb] modify some top comments for image status
move:    chk-and-upd-img.rb -> old/chk-and-upd-img.rb