As a committee member or site admin, I want to have a configurable banner, so that I can quickly show important information for our users

### 📝 Description

This feature would allow the enabling and configuring of a banner that shows important information, like these ones:

This could be useful for multiple reasons, for example:
- NollKIT advising Nollan to read some important information
- Admins warning users when a service is down (or, for example, the next time Slack notifications stop working).
- Other committees or users warning about important deadlines or events.

### ✅ Acceptance Criteria

- [ ] An admin page configuring the banner
- [ ] Configure: type of message (warning, info, etc.). (This can be shown, for example, by setting the background color and/or changing an icon).
- [ ] Configure: message content.
- [ ] Configure: an optional with configurable text and URL.
- [ ] The banner itself, following the rules set above.
- [ ] Rules about who can edit the banner content.

_No response_

No files or tests are named in the issue. First locate the existing admin/settings entry point and the shared site layout, then trace current permission checks. Done means admins can configure the message type, content, optional linked text and URL, while the banner renders accordingly and editing is restricted by defined rules.

Written by the indexing model from the issue text.