Toot visibility icons

[MightyPork]

This is a feature we've been using on glitch-soc for a couple months now and it's proven immensely useful.
I'm making an issue first before spending any time on re-implementing this for a PR, to gauge interest.

They look like this (in the corner):

Since upstream toots have a different layout to ours, we'll need to find a different place for those icons. Maybe the bottom right corner would work?

[wxcafe]

I really like this idea yeah 👍

[renatolond]

I really like this too. I was checking how it was implemented in glitch-soc and it would be awesome to have it brought to upstream. 👍

[Cassolotl]

I think the date/time makes more sense in the bottom right corner anyway - there's usually more space down there.

[wbrawner]

+1 on this. Also it would be nice to be able to click the icon and change the visibility after the post. Might be difficult with federation but at least being able to change the future visibility would be an improvement I think

[trwnh]

fixed by #14123, this issue can be closed