Add banner option for admins

[ElenaStroebele]

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

• I have read and understand the contributing guidelines
• I have checked the existing issues for whether this enhancement was already requested