Add type to broadcast messages

What does this MR do?

Part of gitlab-org/growth/product#42 (closed)

Adds two different types of broadcast messages. The type "banner" is the current design and will be kept shown on the top of the site and the type "Notification" will be shown on the bottom. To still retrieve all current available broadcast messages, we kept .current.

I've chosen to split the methods to act like scopes and make it easier to deal with the cache keys. This is just the Backend MR, Frontend will follow.

Adding the type field to the API will be done in a follow-up MR

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

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