Configurable pinned build duration

[dzink]

Sometimes I only want a pinned build to last until the PR is closed (via merged or otherwise) so that clients can review it, but then I don't care about it anymore.

Currently, we have a number of pinned builds which don't need to be pinned but are still pinned. This is giving us a system where everything is pinned, therefore nothing is really pinned. It would be convenient to have a selectable duration for pins - forever/until PR closes/etc so that outdated pinned builds can be reaped again.

[AlexU-A]

I think it would also be good to see a list of pinned builds, if possible.

[ElusiveMind]

So the conditions of the pins could be (or options):

1. Pin until pull request is closed or merged.
2. Persistent Pin (endless)
3. Time based (end of sprint or production push) - Would be awesome to leverage JIRA here but that's kinda narrow-casting

#1 would be the default pin.