Feature request: Allow selection for local-only posts where supported

[giromide]

In the post dialogue, please consider a second selection next to existing "Post visibility". This could be called "Post federation" and would feature choices of "Local-only {instance name}" or "Federated", and it would only be available to those accounts on instances that support this feature, e.g. Hometown-forked instances.

[amdavidson]

This is a bit niche as it’s not a feature from mainline mastodon but for those of us on @dariusk’s Hometown instances it would be great to have.