Add support for Forgejo backend

[philiprenich]

For context and discussion, with a potential solution, see #198

With Foregjo now being a hard fork of Gitea, a separate change is needed in their codebase for this to be supported.The Gitea patch from the above linked issue should hopefully be portable to Forgejo to expedite this, if someone has the time and knowledge to do that.

This issue is a way to track if that work happens so support here can happen once it does. And as a way to point someone to a place to contribute if they're able.

There's no expectation for work to be done in Sveltia prior to Forgejo's API supporting the needs and issues laid out in #198.

[philiprenich]

Gitea now has the correct API to be supported by this project. Hopefully it can be used as a template for Forgejo to be supported.