Datastore improvements (entities not backwards-compat)

Split Feed into Feed and FeedMeta
Removed unused properties from Article, Entry and EntryMeta
Removed unused indices
Added timezone support for CET
Restored Article.Fetched (instead of Article.Time)
Fixed feed title lookup bug, if the URL of an existing RSS feed is
provided
Added additional logic to disregard mismatched (removed) dataset
properties
FIXME: FeedMeta and Feed must still be migrated
Added a simple migration system

Added migration of Feed to Feed and FeedMeta
Fixed UpdateFeed bug
Fixed SetAttribute migration bug