make ResourceCollection behave more like a Hash

unneeded complexity is unneeded
Merge pull request #2078 from Daxter/refactor/resource_collection

Refactor ResourceCollection
improve belongs_to error message

now it won't throw a huge, useless error message

closes #1367
[Added] option to hide new record link for has_many form and changed documentation.
Merge pull request #2134 from developer88/master

Hide new record link for has_many form
prevent assets from being registered multiple times

JavaScript and CSS registrations are now stored inside a Set and a
Hash, respectively, to ensure that no duplicates are created.
Merge pull request #2139 from Daxter/bugfix/1845-asset-registration

[1845] prevent assets from being registered multiple times
refactor menu item attributes

This commit merges `menu_item_menu_name` into `navigation_menu_name`, as
they both appear to have the same purpose. By doing so, this seems to fix a
previously unreported issue of belongs_to nested resources. Now, the menu
items for those nested resources show up in a unique navigation menu
specific to the parent resource.
Merge pull request #1987 from Daxter/refactor/1967-menu-updates

Refactor [1967] Menu Updates
Index results label fix
Merge pull request #2135 from mindhalt/index-results-label-fix

Fix translations for languages with multiple plural forms