Adding a Content-Security-Policy Header

### Current Behavior

No CSP present:

### Proposed Behavior
https://github.com/DependencyTrack/frontend/blob/master/docker/etc/nginx/conf.d/default.conf#L4
`add_header Content-Security-Policy "default-src 'none'; img-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; font-src 'self'; connect-src 'self'; manifest-src 'self'" always;`

### 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