Add support for image thumbnails in Activity feed

The Activity feed redesign from #49403 (closed) included the following point:

For comment rows, a short preview will be shown inline. If any images happen to be fetched within this preview, they will be shown as small thumbnails which will open the image in a new tab on click.

This is not yet supported by the backend or the frontend, so we decided to leave it for a future iteration. This issue will track that addition.

Solution

Clicking on a thumbnail will open the full image in a new tab.

The specs for the thumbnails will be:

• Max height: 64px
• Border: 1px $grey-200
• Border radius: 4px
• Margin to comment body: 16px
• Margin between thumbnails: 8px