Add a prompt to remind you to add alt text to media

This is a common feature in several Mastodon clients and even BlueSky, which is to prompt the user before posting media without alt text. I always make a habit of this, so this is even useful for me so I don't forget.

Of course, if you really intend to not use alt text you can skip the reminder. This dialog can be turned off if you find it annoying.

(See comments for an up to date screenshot)