[FEATURE] Alt text support for image posts

[noClaps]

I think this would be a huge step forward for accessibility, plus being able to add additional context to an image post without having to write it all in the title or in a comment would be really nice to have. The alt text could be set to the alt and title attributes, and also appear as an image caption below the image.

I think the best implementation for this on the frontend would be something like:

<figure>
    <img src="{image source}" alt="{alt text}" title="{alt text}" ... >
    <figcaption>Alt: {alt text}</figcaption>
</figure>