content views - adding the 'filters' placeholder back in to routes/controller

During a recent merge in to content_views branch, the filters
code was removed.  This was on purpose, because the pre-pulpv2
filter logic was removed.  That said, in the content views
branch we had added a placeholder in to the routes/navigation/
controller/view to support a filters tab for a content view
definition.  Part of that logic was removed, but not all of it.
Adding it back now... since it will be needed when filters are
introduced.

Without it, user will get an error trying to open a content
view definition.
content views - update sortElement asset to pull from alchemy vs converge-ui
content views - if task is nil, publish failed

A content view is created when a view definition is published.
During that process a task status is created; therefore, if it
is nil, then the publish somehow failed before reaching that point.
content views - updates to support retry on refresh/publish failure

This commit contains changes so that 'retry' of a failed
publish or refresh works properly.

The following are a couple of basic scenarios:

- A user attempts to publish a content view definition.  During
  the publish attempt, a 'version 1' of the view will
  be created in the library.  If generating that version (e.g.
  creating the repos) fails, version 1 will be marked with
  an 'error'.  At that point, the user can click "Retry" to
  attempt it again.  During the retry, a new version
  (e.g. version 2) will be created.
  Note: the behavior for 'publish retry' is essentially the
  same as a refresh.

- A user attempts to refresh an existing content view.  During
  the refresh attempt, a new version (version N+1) will
  be created in the library.  If generating that version
  (e.g. creating/cloning/syncing repos) fails, version
  N+1 will be marked with an 'error'.  At that point, the user
  can click "Retry" to attempt it again.  During the retry,
  a new version (e.g. N+2) will be created.

As can be seen by the above, both publish and refresh generate
"a version" of a view.  The only difference being that
publish is always version 1 and refresh is N+1.  As a result, this
commit also includes changes in how the status is displayed to the
user.  Rather than say "publishing/refreshing/publish failed...",
the user is given a generic status of "Generating version...",
or "Error generating version...".

These changes also simplify the polling for the status
of these activities.  Rather than ask for 'publish' tasks
and 'refresh' tasks, we simply ask for any 'pending' tasks.
content_views - remove unused publishing methods from content_view.rb