Add spinner for long-running operations in frontend

### Current Behavior

If you download a BOM for a project with many components (>10,000 in my case) it can take a long time (more than a minute) for the download to start. Meanwhile there is no indication that something is happening in the background and users are includes to click a second or third time.

### Proposed Behavior

Add a visual indicator in the frontend, e.g. a spinner, whenever an operation that could take longer is triggered.

### 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