Shorten story title input placeholder

Remove "100 characters or less" from the story title input
placeholder. With the title input in the header, the shorter input
length causes the longer placeholder to get cut off.

Addresses #578
Add a label for the section title input

This is to try to disambiguate this input from the story title
input.

Addresses #579
Merge branch 'develop' into 579-rework_workflow_steps
Close old alerts when the publish button is pressed

Alerts triggered by the publish button should be cleared each
time the publish button is pressed.  Otherwise, the error for an
unsuccessful publish will remain, even after the missing fields
have been added and the story is successfully published.

In order to get this to work correctly, we need a mechanism for
views to track and close ``AlertView`` instances created as a
result of triggering an ``alert`` event on the event bus.  This
commit adds two changes that support this:

* ``AlertManagerView`` triggers a ``show:alert`` event on the event
  bus when it displays an alert.  The event parameters include
  the ``AlertView`` instance.
* The ``alert`` event now accepts an optional ``alertId`` property
  as part of the ``options`` argument.  This ID is bound to the
  created ``AlertView`` instance.

With these changes in place, views can provide alert IDs when
triggering ``alert`` and listen to ``show:alert`` and check the
view's ID to see if it is the view corresponding to the alert
triggered by the view.

Add an ``options`` argument to the ``initialize()`` method of
``PublishViewBase`` and its subclasses in anticipation of more
recent versions of Backbone that don't automatically extend
the view's ``options`` property with the options argument to
``initialize()``.

Addresses #579
Show story validation errors or warnings, but not both

Based on my first usability test, the user didn't see both the
error for the missing title and the warning for suggested fields.
When validation fails, the user should just see the most pressing
alert message, the error.

Addresses #579
Fix auto-setting of featured asset

Under some conditions, the asset model instance syncs with the
server before the event listener for the ``change:content`` is
wired up, causing the ``change:content`` event to not be fired,
and the featured asset to not be set.

In ``FeaturedAssetView.handleAddAsset()``, check if the ``content``
attribute is set, and if it is, immediately set the asset as the
featured asset instead of wiring up an event listener.

Closes #908