906859-import-messages - cleaned up error messages for both import and delete manifest

Minitest is skipped until non-private manifests can be created to excercise the code and be checked into git
Destroy default content view on cascade when deleting environment

Also validate the default content_view explicitly. Here is what happened
without this changes:

1. Create environment (default content view and content_view_version
was crated)
2. Delete the environment (`content_view` stayed there because
`:dependent=> :destroy` was missing)
3. When creating an environment with the same name, the content_view
was not created, because another content view with the same name
already existed. However, because the content_view was being saved as
part of saving content_view_version, the exception was not thrown (and
the content_view_version was saved successfully without association to
any content_view)
4. When promoting some time after this incident (that happened
unnoticed), "nil pointer exception" occurred while promoting, because
the default content view was not there.
Merge pull request #1704 from thomasmckay/906859-import-messages

906859-import-messages - cleaned up error messages for both import and delete manifest
Merge pull request #1743 from iNecas/default-content-view-dependent-destroy

Destroy default content view on cascade when deleting environment
Worked on content view search
Content Search: created a content view comparison
Content Search: Refactored content view comparison

Refactored content view comparison by breaking out code into classes
Content Search: refactored code by creating module namespaces
Content Search: Fixed metadata row in view comparison
Content search: fixed link and removed duplicate code
Content search: added pagination to content view comparison
Content search: Fixing product_repos method name
Content Search: moving files to app/lib
Content Search: not displaying total packages per product on cv comparison
Content search: fixed search modes for cv comparison
Content Search: tweaked product row in cv comparison
Content Search: showing cv filter for package search
Content Search: fixes to existing code

Filtering product and repo searches by default content views and added a new
ProductRow model.
Content Search: filtering views by search mode