branding image should be deleted after new image uploaded

[cruvie]

Describe the bug A clear and concise description of what the bug is.

To Reproduce Steps to reproduce the behavior:

1. deploy with docker-compose

2. upload brand image twice

3. check path /answer-data/uploads/branding

4. See two images include the previous one

Expected behavior the previous image should be deleted

Platform (please complete the following information):

• Device: Desktop
• OS: macOS
• Browser edge
• Version 1.2.0

[kelvinkuo]

Is this so important? Changing logo is a very low frequency operation. So disk wasting is not too much. Maybe there will be a feature like "recovering the last logo" some day and need keeping the last logo on disk.

My personal thoughts.

@cruvie

[Octobug]

This is relevant to #300 , which provides a management UI for uploads.