Add spinner for long-running operations in frontend

[sithmein]

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

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