Add ability to maximise images in chat

[marksgraham]

Description

Allow images to be maximised in chat. It was sometimes hard to make out the details of displayed images, especially those containing text.

Verification

[jimbojw]

Thanks Mark, I see what you're going for here. I'm still onboarding to the project, so I can't speak with perfect authority yet overall, but I have a compartmentalization recommendation.

Your proposal to attach an object to the document.body is fine for a quick solution. I think we can streamline it a bit.

Currently you're creating DOM elements inside the chat message's renderModalToBody() method and giving them inline style and behavior. Instead, I'd recommend making a new, separate FullscreenImage component. This object would have all the SCSS and callbacks (close button etc.) in its fullscreen_image.scss and fullscreen_image.ts respectively.

Then, in the chat_message.ts, you should be able to create a new FullscreenImage() and attach that to document.body to get the same effect.