Gitea integration + refactoring of current git server

[meffmadd]

Is your feature request related to a problem? Please describe.
While our git server implementation works, it would make more sense to have a dedicated service (i.e. Gitea) handle the git workload (for scalability considerations, features, stability, etc.).

Describe the solution you'd like
The grader service still has to act as a proxy because we create repositories on the fly if they do not exist and for authentication and authorization purposes. Furthermore, if the grader service acts as the proxy, the git remote URLs can remain unchanged regardless of what git server is configured.

To be able to integrate the changes, the current git server will be refactored into extensible and simplified request handers (closing #71 and #78).

Another option is to use gogs.