Use thumbnail URL instead of channel data for storing the thumbnail

[compulim]

Version

master or 4.5.2

Describe the bug

Today, we use channel data to save the thumbnail.

We should update it to use thumbnailUrl instead.

To Reproduce

Steps to reproduce the behavior:

1. Go to playground
2. Upload a photo

Expected behavior

Look at the traffic:

• It should use attachments[].thumbnailUrl to keep the data URI
• It should survive F5
  • Please add a test, I don't think we have a test for survivability

Additional context

We have tests today to verify the thumbnail image quality can be tweaked.

[Bug]