Show custom domain somewhere in editor

[StachuDotNet]

e.g. on https://darklang.com/a/ops-presence, I'd like to see https://presence.darklang.com somewhere.

[StachuDotNet]

This was a matter of discover-ability for me - I had access to the canvas, but had to search through Dark code to find the corresponding URL.

It's unlikely anyone would forget their single custom domain, but in an org setting it may matter more.

[pbiggar]

we could load it in intial_load, it's an easy DB lookup.

Though I want to move away from initial_load being the be-all and end-all to get everything, so maybe we could make a request to an API for this on the dark-editor canvas.

[StachuDotNet]

Sounds great as an endpoint in dark-editor!

[StachuDotNet]

Closing #3696 in favor of this.
From that Issue:

Users can click on the ... menu in handlers to get a link to the handler, but we don't have that link presented anywhere more obvious. We should add a link, maybe in the menu?

[StachuDotNet]

Especially as this relates to darklang.io domains eventually being used, and allowing multiple domains for a canvas (routing different routes to different domains), this is a bit trickier and should be spelled out before being an Issue, so migrated to a Discussion here.