Allow broadcast messages to be closed
Description
Broadcast messages, such as in the screenshot below, are configurable and can appear on instances of GitLab - including GitLab.com. Once a user has seen a message, it should be reasonable for them to close/remove the message so that it's not displayed again.
Proposal
- Have a close button on the broadcast message
- If a user closes a message, it should not displayed again on subsequent page loads