Image attachments should be rendered with correct dimensions

[petyosi]

Right now, images are rendered without a height attribute, which causes jumps in both message lists. Backend will soon start returning versions and dimensions for the images, which we can take advantage of.

Acceptance Criteria

• img elements are rendered with width and height attributes to avoid layout changes
• optionally, the developer should be able to specify the preferred version of the image (TBD with backend) - prop on the Channel should work.

Definition of Done

• implementation
• documentation for the image version property
• doc comments