Author Validations

# References:
* https://www.pivotaltracker.com/story/show/76096156
* https://www.pivotaltracker.com/story/show/75982222

# Major changes:
* AuthorsTask has been moved from StandardTasks to its own engine and named `PlosAuthorsTask`, so that it can be used as a pattern going forward.  (This includes all the author ember components that used to live in the main application.)

* Authors model still exists, but only containing very sparse attributes (`first_name`, `position`, `paper_id`, etc.)

* All specific Author related attributes now live on `PlosAuthor`

* TaskType creations now is smarter and will detect when an existing TaskType has changed or when one has been removed

* New component (`field-with-errors`) introduced to display field level errors in a common way

* Validator (`AssociationValidator`) introduced so that related models can be easily validated and errors bubbled up correctly

* Subscriptions/Notifications related to the `PlosAuthor` has now been pulled into the `PlosAuthors` engine

# Special notes:
* Making changes on existing authors while the Task is marked complete is out of scope for these changes.  See pivotal card #81186594

--- AC + CT
The migration which creates the new table still needs to live in the engine, otherwise future users of Tahi who don't include the `PlosAuthors` engine will still have the table.

I agree that it's not immediately urgent, though. I created [a chore for this](https://www.pivotaltracker.com/story/show/81304128) so we can track this separately.