Revert "WIP: display a link to the feed in the subscribe job alert, if the job finished successfully."

This reverts commit 7db5d3c6a65c4c36b24ca747d154ac04425a2e69.
Added one-to-many association between feeds and subscribe_job_states.

If the subscribe_job_state has state "SUCCESS", its feed_id foreign key must be different from null. If the job has any other state, the feed_id foreign key must be null.

This makes sense because until a subscribe job is finished successfully, we cannot be sure that the associated feed exists in the database.
Rewritten private method that deletes refresh_feed_job_states when unsubscribing from a feed.

It's functionally identical, but more readable now.
If SubscribeUserJob finishes successfully and a subscribe_job_state has been passed, the feed_id attribute of the subscribe_job_state is updated with the ID of the subscribed feed (which is guaranteed to exist in the database, and the user is guaranteed to be subscribed to it).