Swap logrus for clog15/log15

[muhlemmer]

Clog15 is a utility package which attaches a log15 instance to a context. This make it easy for both gRPC and http servers to do request scoped logging. As a personal preference, I find log15 cleaner and easier to work with.

httpauth, forms and middleware are already built with this change in mind. The gRPC server and admin still need to be adapted.

This is a non-essential and long term cleanup effort. No deadline.