Gitea integration support

[lunny]

Gitea is a welcome open source self-host git service. It's better to support Gitea instance on sourcegraph browser extensions. Will resolve go-gitea/gitea#5010

[ijsnow]

We would be happy to add support for Gitea to this repository. However, since Gitea is open source, it would be better to integrate it as a first class integration with Gitea. This means everyone who uses Gitea would be able to benefit, not only those who have heard of this browser extension and installed it.

[sqs]

Duplicate of #343