Add ability for the new dropdown to take a partial and load modal

app/helpers/nav/new_dropdown_helper.rb

app/views/groups/_invite_members_modal.html.haml

app/views/groups/_invite_members_side_nav_link.html.haml

app/views/groups/_invite_members_top_nav_link.html.haml

app/views/groups/group_members/index.html.haml

app/views/groups/show.html.haml

app/views/layouts/header/_new_dropdown.html.haml

app/views/projects/_invite_members_empty_project.html.haml

app/views/projects/_invite_members_modal.html.haml

app/views/projects/_invite_members_side_nav_link.html.haml

app/views/projects/_invite_members_top_nav_link.html.haml

app/views/projects/issues/show.html.haml

app/views/projects/merge_requests/_page.html.haml

app/views/projects/project_members/index.html.haml

app/views/projects/work_items/index.html.haml

ee/app/views/projects/learn_gitlab/show.html.haml

...ec/views/projects/project_members/index.html.haml_spec.rb

lib/gitlab/nav/top_nav_menu_item.rb

spec/helpers/nav/new_dropdown_helper_spec.rb

spec/lib/gitlab/nav/top_nav_menu_item_spec.rb