A post has a relay_count
Adds seed_db script (SEED_DB env variable)
Post's `_author` is set as the `_last_relayed_by` by default

* `_author` and `_last_relayed_by` fields are omitted in query results by default.
Fixes broken scenario test
Corrects Post#getPopularPosts
Feed displays the post's last_relayer's username
Feed shows post's relay_count

* Adds Ability to assign post.relay_count without it persisting to database.
* `_relay_count` is actual MongoDB field
* Creates virtual getter/setter for post.relay_count