Arvados

Things to consider:

• There's a lot of ways a pipeline could change: changes in git, changes to source collections (including Docker images), changes to the actual pipeline template. Is there a key event we pay attention to to notify users? Maybe we just give pipeline authors a button to send out the notifications?
• What does the notification look like? Does it appear in the notification drawer? On pipeline instance pages? What information does it need to include?
• Do we need to provide some kind of pipeline throttling?