Scheduling an issue requires Engineering and Product collaboration

Why is this change being made?

We've been talking a bit about process on the Manage stage. Import has created a helpful doc for their group, but I think it's useful to have some consistency across groups.

We should do this while leaving room for individual groups to figure out the details, but regardless of how we work - Product and Engineering should be in sync, without issues surprising either department. To facilitate this, the MVC to our process feels like making issue scheduling (applying a milestone to an issue) something that only happens after both departments have given an issue an OK.

Example: gitlab-org/gitlab#118 (closed)

How will this merge request impact page speed?

No

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what