Support for local-only posts

[avrhmt]

Is your feature request related to a problem? Please describe.
While not part of the standard Mastodon API, other mastodon forks such as Glitch and Hometown support the option to limit post visibility so that only instance members can view the post, in addition to Mastodon’s main post visibility settings. This is helpful for community instances where users want some of their post to only be visible for members of the community.
Currently, this feature is not supported by Metatext.

Describe the solution you'd like
I would like to see this feature supported by Metatext. Both in allowing the choice in the compose window, and by showing the visibility status of posts in the timeline

Additional context
Hometown’s Wiki page on Local-only posts:
https://github.com/hometown-fork/hometown/wiki/Local-only-posting
An open source iOS client named ‘Tusker’ (in beta) has implemented support for this feature. You may be able to take inspiration from their work (MIT licensed) https://git.shadowfacts.net/shadowfacts/Tusker
I have also included a screenshot of the UI in Tusker’s Compose view: