Reconsider annotation related table structure

[kyoshizaki]

At present, there are three tables related to annotations, snippets, stickies, and links.
In order to be able to give these annotations flexibly to content inside and outside the LePo, fix the table structure as followings.

• Table rename: from web_sources to web_pages
• Table rename: from links to bookmarks
• Column reconsider: bookmarks
  • add: target_id, target_type
  • rename: title => display_title
  • delete: url

[kyoshizaki]

Done!