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
Although I agree with you that the migrations should be split, I think in the interest of time, we leave these as is.  Otherwise, it's going to be a bit of ceremony to split these out now (since they've already been run on staging) with really no perceivable benefit to the app.  Definitely need to remember to put any engine level migrations in the engine folder going forward.  Might be prudent for: https://www.pivotaltracker.com/story/show/72706364