Group post improvements (especially on Akkoma)

[Steffo99]

This patchset adds an alternate way of detecting whether an user is a Group or not on https://github.com/cheeaun/phanpy/labels/akkoma (and most likely https://github.com/cheeaun/phanpy/labels/pleroma too), since the default one doesn't work there.

Additionally, this adds a "Posted in XYZ" header to group posts, similar to the "XYZ boosted" of boosts.

[cheeaun]

Maybe remove the "Posted in" change for now? Groups UI/UX on a social media timeline UI is kinda weird in my opinion and I don't want to put too much effort into it yet (and also translators' efforts).