[chrome/breadcrumbs] migrate to the new platform

[spalger]

As a part of working to make some changes to the way that breadcrumbs work so they are more unified across the k7 design I figured it was a good time to migrate the breadcrumbs to the new platform, and along the way eliminate the angular-specific wrapper (while keeping the angular integration that clears the breadcrumbs via angular router navigation events).

The primary breadcrumbs state is now in the core.chrome service, along with the type definition for Breadcrumb. The following changes were made for consistency with other new platform code:

• ui/chrome/api/breadcrumbs receives the chrome start contract like other ui/chrome/api/* modules
• ui/chrome/services/breadcrumb_state was moved to chrome.breadcrumbs, so:

  import { set, breadcrumbs } from 'ui/chrome/services/breadcrumb_state'
  set([]);
  breadcrumbs.subscribe(...)

  is now

  import chrome from 'ui/chrome'
  chrome.breadcrumbs.set([]);
  chrome.breadcrumbs.get$().subscribe();

• renamed instances of breadcrumbs referring to the stream of breadcrumbs to breadcrumbs$ inline with our loosely held convention.

[elasticmachine]

Pinging @elastic/kibana-platform

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[spalger]

6.x/6.6: 696c49b