Add icon on media that lacks alt descriptions

[dariusk]

Mastodon now has a hint when you are drafting a media post that you need to add alt text. But there is still no indicator on other people's posts that the media lacks a description. Lots of people want to only boost described media.

My plan: add an icon to all media when it lacks a description. I choose this rather than what Twitter does (add an icon when there is a description) because described media should be the default, "pretty" version of posts.

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Done! It looks like this, warning icon in the lower right:

And similar for audio/video:

[dariusk]

Thinking about this more -- it makes sense to also apply some classes so that admins can easily tweak this behavior if they don't like it specifically.

[dariusk]

Current WIP: