I don't fully understand the impact of these API changes, but without
them the status from buildbot upon a successful build was causing gitea
to mark commits with a yellow circle instead of a green checkmark.
Since GiteaStatusPush was based on GitLabStatusPush in the first place,
I tried making it look more like its ancestor -- and now the commit status
in gitea is working correctly.
Apparently, as of buildbot 2.9.x, [reporters need to provide their own
checkConfig() implementation when a service has custom args not supplied
to the superclass](https://github.com/buildbot/buildbot/pull/5571). If
we don't do this, checkconfig will fail.
