Embed dashboard groups and implement new URL routing

[sultan-duisenbay]

Apologies for the large PR. I wanted to make sure that group view was embedded and worked nicely with URLs too.

The end result screenshot:

Major changes

• Add several routers to detect the nesting level and resource types
  • testgrid-root-router to detect index page or group view page or pass parameters to next level
  • testgrid-dashboard-router to detect dashboard resource level or pass to next level
  • testgrid-tab-router to detect tab resource level
• testgrid-data-content -> testgrid-dashboard-content
• Created a top level container element called testgrid-group-content which will render either the group view or the testgrid-dashboard-content.
  • This element will be added to DOM irrespective of the URL resource level just with different properties. The overall view and content will be tailored according to those properties.
  • testgrid-group-content has its own tab bar to switch between dashboards.
• Improved the existing tests and added some tests for testgrid-group-content element.

[google-oss-prow[bot]]

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