Clock the time needed to prepare 1.2.3

This is required by a funder to have a comparison of the progress made on RM over 2015.

This corresponds only to preparing, build, and testing the ISO and announcing the release. Both the natural and working time.

It does not cover the previous review and merge work. But if anonym wants to clock this as well, that would surely be interesting.

Parent Task: #8391

Original created by @sajolida on 8390 (Redmine)