Allow to link relatively to another instance in a new posts' URL

[db0]

Is your proposal related to a problem?

Currently when I post a URL for a new post, I am forced to use a full absolute path (i.e. starting with http(s)://). However when cross-posting between instances, this absolute link prevents someone from commenting or subscribing to that community

Describe the solution you'd like.

Allow posting relative links in the url field. This will allow one to link say to /[email protected] and whoever clicks this link will be transported there through their own instance, allowing them to subscribe and comment normally.

Describe alternatives you've considered.

I couldn't think of anything else that works

Additional context

Cousin of #1227

[orientalsniper]

This is a godsend if you have another friend on another instance and if I share my link, it won't work for him.