Improve pipeline table to add meaningful metrics

[jechols]

Pipelines have started_at and completed_at fields, and while those can be helpful, they lack a lot of rather useful details. Is the pipeline still running? What's the total duration of jobs that have run? Is completed_at blank because the pipeline is in process or because the jobs had to be canceled?

• Add a status field onto pipelines, even if it's just "pending", "started", and "complete"
• On completion, sum up the total duration of all jobs on a new total_duration field so we can see how much time was actually spent, not just the start and end times

There may be other useful data to add, too. Think about this from the perspective of what would be nice to see in a "jobs overview" kind of UI. What would be useful when looking at a list of pipelines, and what would be useful when looking at their jobs?

[jechols]

See also #159