Collect build time for submissions

[babywolfh]

Build time information is important part of the process and one of the main criteria in defining the order of accepting submissions. We need to have this info in IRIS and add it to the results of list submissions API and submission details API.
Build time is a time between submission is created and the time of the last update. There is no need to modify jenkins for that as IRIS can just store creation and update time. There could be some corner cases in determining build time, e.g. when build is restarted build time should be reset.

I set priority to high as repa already shows build time. So we need this feature to provide similar user experience for repa users when working with IRIS.