Add announcement for category page

[harriebird]

Proposed changes

Add announcement form on category page. To achieve this, changes are made to the models Change and Announcement. This solves issue #11106.

Checklist

• [ ] Lint and unit tests pass locally with my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added documentation to describe my feature.
• [ ] I have squashed my commits into logic units.
• [ ] I have described the changes in the commit messages.

Other information