Performance: Improve image loading in `UserAvatar`

[umpox]

Description

Goal: Reduce time spent loading images on every page

Reason:

• We use a lot of GitHub User Content images at a large size, which we then scale down
  • We could use smaller images and have the same impact
  • Note: We also support custom images from other sources.
• Best solution might be to always ensure we are lazy loading?

Note: @jasongornall Found a security issue with our current images. Might affect this work: https://sourcegraph.slack.com/archives/C1JH2BEHZ/p1658845789402819

A/C

• Faster loading of images in UserAvatar

[sourcegraph-bot-2]

Heads up @taylorsperry @jasongornall - the "team/frontend-platform" label was applied to this issue.