Update milestone dates dynamically based on estimates

[sanity]

The completion dates of milestones should be updated automatically based on the total estimates of the issues assigned to those milestones.

I think we have to be smart about how milestones are prioritized though. This could be done through dependencies, or alternatively just through some text like:

priority:5

In the description of the milestone. To determine milestone completion dates I think an algorithm like this might work:

Firstly assign the priority of the milestone to each issue that is part of that milestone (if we have dependencies then we'll need a first-pass to assign priorities based on these)

Secondly, for each developer sort all of the issues assigned to them according to priority (so highest priority gets done first). For issues with the same priority, those for the same milestone should be grouped together.

Lay these out on a timeline considering 5-day work-week etc. so we know start and end dates for each priority.

Then for each milestone, find the earliest date when all of the issues for that milestone are (estimated to) be completed (remembering that multiple developers may be working on the issues for any given milestone), this will be the milestone completion date.

[sanity]

This is basically done, or already in other issues