Alt text for embeds

[sevenc-nanashi]

Description

Add alternative text for embeds when user disabled "Show website preview info from links pasted into chat"

Why is this needed

Many bots are using embeds for response, but user cannot see embeds if they disabled "Show website preview info from links pasted into chat".
So user will see like this:

You can send message with content like this:

But it will be annoying when it's enabled:

So I suggest placeing alternative text.

This may be useful with #3347 like this:

[advaith1]

duplicate of #3226