Promote project milestone to group milestone

• A scoped down version of https://gitlab.com/gitlab-org/gitlab-ce/issues/35833 to just get the functionality of milestone promotion.
• We will use https://gitlab.com/gitlab-org/gitlab-ce/issues/35833 at a later time to finish up the redesign of the milestone pages.
• Promotion is analogus to label promotion in that all project milestones with the same name will be merged.
• For this issue, UI should be the same/analogous as promoting a project label.

Flow

• There is a button to to promote a project milestone on the project milestone list page. E.g. https://gitlab.com/gitlab-org/gitlab-ce/milestones. Promote to Group Milestone.
• Once you click on it, there is a confirmation dialog. Promoting this milestone will make this milestone available to all projects inside this group. Existing project milestones with the same name will be merged. Are you sure?
• Once you confirm, you are navigated to the group milestone list page. E.g. https://gitlab.com/groups/gitlab-org/milestones. (This is different from the label promotion flow.) There is a banner across the top telling you that the promotion happened. Milestone was promoted to a Group Milestone

Out of scope

• No changes to the visual design.
• The dashboard, group, project milestone list views all remain the same. None of those are changing.

Similar existing label promotion flow