Captions and alt text for images

[sidemt]

Feedback from Abbey:

We need a way to add captions to images for a11y (again, this is critical IMO). Also I assume authors will be able to add alt text to their images?

[sidemt]

I guess we can implement this by:

• Showing fields to input caption and alt text
• Convert them into HTML

so we don't need new attributes in the backend.

It looks like this in Ghost: