Content Views Rework: prepend content_view on filter models, update routes

This commit addresses the following:

1. Rename the filter models/schema, prepending 'content_view_' to each.
   This change is to be consistent with other models
   (e.g. content_view_environments) as well as to ensure that there is
   never a name clash with the core or future models added to katello

2. Update routes to also account for the above change.  For example,
   rather than define:
   GET /filters/:id
   defint:
   GET /content_view_filters/:id
Merge pull request #3764 from bbuckingham/cv-rework-filters-rename

Content Views Rework: prepend content_view on filter models, update routes