Alt text for image posts

[glibg10b]

Requirements

• This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• Please check to see if this request (or a similar one) already exists.
• It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

When posting an image, we have the option to add a body of text, just as with text posts:

However, unlike with Markdown images (![alt text]()), there's no option to add alt text. This leads to everyone using the body text for image transcriptions, which makes this text visible to everyone:

I suggest that a separate "image transcription [optional]" field be added to the post creation form.