chore(feeds): feed_items table to store feed items (#16)
Some RSS feeds are technically invalid but still want to be fetched. In
these cases we want to make sure we can still fetch them and properly
handle when the feed item does not contain correct tags for dates.
Previously we used `published` and internally `last_digest` to determine
if a feed has already been sent to the user. Well some feed items do
not contain `published` so we would always send the feed item to the
user in their email digest.
Now we are storing the feed items we have fetched in a separate table
along with some of its metadata in order to properly mark a feed item as
"already sent." Now it doesn't matter if the feed item has the correct
tags or not, we will try to fetch the items, send them to the user, and
them mark them as already sent.
