Dropped support for `DRONE_*` environment variables in pipeline steps. Pipeline meta-data can be accessed with `CI_*` variables.
- `CI_*` prefix replaces `DRONE_*`
- `CI` value is now `woodpecker`
- `DRONE=true` has been removed
Resolve some todos in server/model:
* Move persistent queue into its own package
* Create Types: StatusValue, SCMKind, RepoVisibly
* Rename struct Repo fields: SCMKind, IsSCMPrivate
* move api code to server/api
* move grpc server for agent communication to server/grpc
* move server.Config to server/config.go as it is used by both server/api and server/grpc
* move shared code used by server/api and server/grpc to server/shared