Local posts support for other fediverse backends

[Steffo99]

From @graue in #657 (comment)

[VyrCossont]

GotoSocial is adding these as well. Technically, we've had them for a while, but with the upcoming 0.17 release, they'll be using the same local_only post parameter as Hometown.

[Steffo99]

Also referenced in: superseriousbusiness/gotosocial#3309

[sgrigson]

In case you're curious for UI reasons (not that this is necessarily the best design) this is how Glitch handles it:

So you can have 'Local-Only' Public posts, Local-Only Unlisted posts, Local-Only Followers Only posts.

[graue]

Here's how it's presented in Hometown. Essentially identical to Glitch.

[Steffo99]

@sgrigson @graue Thanks for the screenshots!

[sgrigson]

@Steffo99 Got a PR for GoToSocial here:

#1047

[sgrigson]

As mentioned in the PR, this method would work seamlessly for Mastodon-Hometown as well, as Hometown does local-only posts the same way.

This PR only focused on adding the functionality for GTS, however.