Add support for Akkoma

Summary

Adds support for Akkoma. Akkoma sucks and you shouldn't use it. But in case you are, this way of supporting it makes sense.

We basically treat it like it's bleeding-edge Pleroma with a +akkoma fork. This is achieved by detecting if the instance is Akkoma, then replacing the version with this hardcoded string: 2.7.2 (compatible; Pleroma 2.4.5+akkoma). We'll always target the latest version of Akkoma (similar to how we deal with Soapbox). This way it's basically Pleroma and we can toggle features on and off as expected.