Group-level broadcast messages

Problem

GitLab offers the ability to display broadcast messages across an instance to communicate important messages.

GitLab.com users don't benefit from this feature, however, since Messages is currently an instance-level feature and only available to admins.

Proposal

Implement group-level broadcast messages:

• Allow a group Owner to set a broadcast message in the settings for the group.
  • If possible, duplicate instance-level functionality.

When both instance and group level broadcast messages are present, handle them like we do multiple instance-level broadcast messages: "If more than one banner message is active at one time, they are displayed in a stack in order of creation."