This commit introduces the following components:
* DBreadcrumbsContainer - The wrapper template-only component,
which renders all DBreadcrumbsItem components on the page.
* DBreadcrumbsItem - The component that registers a LinkTo
for the breadcrumb trail. The breadcrumb > trail > will
show based on the order these items are rendered on the page.
* BreadcrumbsService - Manages the DBreadcrumbsContainer elements
on the page via DBreadcrumbsContainerModifier.
* DBreadcrumbsContainerModifier - Handles registering DBreadcrumbsContainer
elements with the BreadcrumbsService and deregistering them.
For now, we will only use these breadcrumbs in the admin section
of Discourse, and this initial commit only uses them in admin/plugins.
This is heavily based off of
https://github.com/Bagaar/ember-breadcrumbs,
but will be further modified for our needs.
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