Rename router model attr model to content_type

To clarify its purpose
DRY repeat code
Clarify route method intentions

More semantic names, mark as private
Encapsulate Backbone.history.start
More intuitive name for private function
Move stripping of trailing slash outside of router
Implement router.urls model

- Refactored all concerned views to use new model
- Added pluralize_string view helper
- Made router model’s attributes to use consistent naming and style
(exclude_shared => shared)
Make UrlsModel's function names more semantic
Use UrlsModel for search
Merge pull request #1537 from CartoDB/easy-router-urls

DRY'er construction of new-dashboard route URLs