Add "Burndown" Chart

[adewes]

Sprint planning requires to estimate how much time is still required to finished remaining issues in a given sprint.

To facilitate this, we add time-estimate labels to issues, which can be parsed by Gitboard. We want to use this information to generate a so-called burn-down chart (https://en.wikipedia.org/wiki/Burn_down_chart), which will show to the user if it is realistic to complete the sprint tasks on time.