WIP setting up paperclip and writing a few tests

Now with s3!
[#67041100 #67475832] Rename feed_items -> index

Also fixed some rubocop violations.
[#67041100] Test attaching an image to a feed item

Also includes fixing tests and the database cleaner
[#67041100] Finish testing paperclip images

Also includes:

- test for the most_recent scope
- updated_at now included in the feed item serializer
- since_field is now :updated_at
- possibly some other stuff