Ability to add alt text to images

[FitikWasTaken]

Describe the feature

Right now you can view alt text (if you'll click on the image), but you can't add your own

There's an example from Moshidon, Mastodon app about how adding alt text looks there for inspiration, you can click on pen icon below the image and add it there

I think it's pretty important, as alt text can help visually impaired people on the wider Fediverse(not just on MBin)

Additional context

Preferably, you should see the image while you're adding alt text, as it's easier to describe it that way