Wording on modal and banner for project promotions of milestones and labels

Requirement 1: Fix modal title and body

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/41895, the wording/styling should be:

Promote ~Discussion to group label?

Promote 9.5 to group milestone?

Promoting [label-title] will make it available for all projects inside [Group name]. Existing project labels with the same name will be merged. This action cannot be reversed.

Promoting [milestone-title] will make it available for all projects inside [Group name]. Existing project milestones with the same name will be merged. This action cannot be reversed.

Requirement 2: Fix banner

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/41895, the wording/styling should be:

Discussion promoted to [group label](link-to-group-label)

9.1 promoted to [group milestone](link-to-group-milestone)

Requirement 3: Fix EE promote milestone modal

Promoting [milestone-title] will make it available for all projects inside [Group name]. Existing project milestones with the same name will be merged.

[New Paragraph] 
Group milestones are currently missing features such as burndown charts. You will not have these features once you've promoted a project milestone. They will be available in future releases. 

[New Paragraph] This action cannot be reversed.

Currently, these are on GitLab.com: