Milestone progress percentage should be configurable

The milestone overview now shows completion percentage based on the number of closed issues. This does not tell anything about the progress as we always pick the small / bug issues first, which do not have that much weight.

So, we can not say anything based on this percentage. The way this percentage is calculated should be configurable per project, and have options like:

• based on weight
• based on spent vs. estimated time
• based on team volatility
• based on closed issues

For the last one (current behavior) the progressbar could even have multiple colors, based on the status of the issue:

• Unstarted Issues (open and unassigned) => yellow
• Ongoing Issues (open and assigned) => blue
• Completed Issues (closed) => green