Measure how long build targets take in Harbormaster

Summary:
Ref T1049. This keeps track of how long a build target takes to execute in Harbormaster and displays it in the build view page. I'm not sure whether "Started" is really that useful once the target has completed?

Also, I change the name of the time taken depending on whether or not the target has completed; if it's still in progress it's called "Elapsed" and if it's completed then it's "Duration". The primary reason for this is that "Duration" sounds like post tense, whereas "Elapsed" is current tense. I'm not sure whether this is okay or not?

Test Plan: Ran a Sleep build step and saw the target dates / times appear correctly.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: talshiri, epriestley, Korvin

Maniphest Tasks: T5824, T1049

Differential Revision: https://secure.phabricator.com/D10174