Add a starting date to milestones

Description

To be able to scope a sprint, body of work or milestone a starting date helps clearing up when something is to be worked on and when not.

A starting date also allows you to report / graph data about that particular milestone. Without one, you'd need to supply one.

Proposal

We add an optional starting date to milestones.

Links / references

cc @pedroms @smcgivern @dzaporozhets @gl-product

Status changed to closed

moved: https://gitlab.com/gitlab-org/gitlab-ce/issues/23704