Add selection for broadcast notification type

What does this MR do?

We separate between banner and notification broadcast messages. For both types we render a preview when creating or editing them, although notifications don't have different colors.

Part of gitlab-org/growth/engineering#2601 (closed)

Documentation will be updated in a separate MR

Screenshots

Creating / Editing

Listing

How it looks on a page

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