Feature: estimations

[stijn-dejongh]

Add estimations (in hours) to activities.

Enable users to add an estimation to each activity. Add an overviewpage (or add this in the timesheet view) to show the difference between the estimated time, and the actual time.
The first implementation can be naive, future implementation might want to make note of higjh, middle and low estimations.
If possible add a way to display estimate accuracy over time, to see whether the actual time taken converges towards the estimate.