Add banner option for admins

### Current Behavior

Currently there is no implementation for a banner in DependencyTrack.

### Proposed Behavior

Banners allow to propagate important status messages, as well as useful information.
The administrators should be able to configure a banner in the "Configuration" section within "Administration".
One should be able to dismiss a banner.

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/frontend/issues) for whether this enhancement was already requested