Deleting a feed deletes its important entries

[lemon24]

They should likely be moved to an "Archived" feed (mentioned in https://github.com/lemon24/reader/issues/96#issuecomment-460077441).

• Old entries in the archived feed should not be deleted (by #96); if they remain important, they won't.
• The entry source (#276) and original_feed_url should be set to the to-be-deleted feed (if not already set).