Add constraint for milestone dates
In https://gitlab.com/gitlab-org/gitlab-ce/issues/58325#note_147502744 we see that milestone dates can go up to 5874897 AD. This is silly; we're not the Long Now Foundation.
We should probably clamp these to four-digit years, which is still silly but won't blow up like this. We can add a model validation, and we might be able to use https://github.com/nullobject/rein to create a check constraint in the database too.