Task groups

[eliperelman]

This page is synonymous with https://tools.taskcluster.net/groups/e4MvyI2FSjqEmLjZ1mUbYw. In order to address UX concerns, this view will no longer be unified with viewing tasks; it is only concerned about the group itself and its actions.

The task group will be paginated, most likely in pages of 100 tasks. We will also do a server-side filter if specified, and we are aware that this will initially lead to inconsistent page sizes.

This should fix:

• two 3000+ task taskgroup inspectors hang nightly taskcluster-tools#271
• Inspector creates a task and a group listener taskcluster-tools#353
• don't scroll current tasks off screen taskcluster-tools#285
• feature request: filter on label taskcluster-tools#284

This issue should also involve the GraphQL task subscription.