Allow alt-text length to be customisable

[nullishamy]

Closes #2689

I do not have the resources to test this, but I have followed the same procedure as the toot length customisation, so I do not see why it would not work.

[soatok]

This looks like a sensible and self-contained change. It would be Immediately valuable to many people, too.

[kescherCode]

Oh, good change! This is very close to how it's in Catstodon, so I'll go ahead and approve (as a LGTM) :3

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions]

This pull request has resolved merge conflicts and is ready for review.

[sneakers-the-rat]

I like this change a lot, and have proposed something similar on our instance before glitch switched to the new way of serving the max_characters config (though some people said it might be undesirable, as it encourages alt text to be overlong. I disagree, but just repeating the discourse).
https://github.com/NeuromatchAcademy/mastodon/pull/16/files

One thing that i would add is that it might be nice to add the default value to the .env.production.sample so people know that this is possible to customize this.

[hackdefendr]

Last comment was 2 weeks ago. Can we hear from a maintainer on this?

[arachnist]

The description editor modal for vanilla/non-glitch UI, in app/javascript/mastodon/features/ui/components/focal_point_modal.jsx, should probably be modified as well.