Hide broadcast messages till end of period

What does this MR do?

With the recent marketing message on gitlab.com it came apparent that we don't hide broadcast messages until the end of the broadcast message but just till the end of the browser session. We're using cookies to hide them but didn't set an expire date. This MR fixes this by adding an expire to the cookie to the ends_at date of the broadcast message (which must be set).

I'd say we could then close probably #208673 (closed) since the cookie is persistent till the end of the broadcast message.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team