Allow easy comparison of task durations for a particular version in Spruce

The build variant details page in the legacy UI displays the duration (i.e., total time the task took to execute, once started). Comparing the durations across tasks for a particular version is useful to determine what the bottlenecks for that version are, and give insights into which tasks need further attention for performance.

Exposing the task wait time in addition would help shed light on build variants that are exceptionally slow to execute tasks. Note that this request is for wait time to be displayed even after the task has finished executing, for retrospective analysis.

This information also could serve as an entry point to a more detailed investigation into a task's timing via the Stats page.

Per the comment thread, the task time display should be able to be filtered, e.g., by build variant.