Feeds: Don't forget entries or their unread status

Three important changes:

1) Visited URLs can be marked as "kept" so they will never be discarded due to old age.
2) Feed entries are not discarded from the database until they are removed from the source, and then become too old (six months).
3) Visited feed entry URLs are always flagged as kept, so the (un)read status will not be forgotten.

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