Merge pull request #14476 (taskserver)

This adds a Taskserver module along with documentation and a small
helper tool which eases managing a custom CA along with Taskserver
organisations, users and groups.

Taskserver is the server component of Taskwarrior, a TODO list
application for the command line.

The work has been started by @matthiasbeyer back in mid 2015 and I have
continued to work on it recently, so this merge contains commits from
both of us.

Thanks particularly to @nbp and @matthiasbeyer for reviewing and
suggesting improvements.

I've tested this with the new test (nixos/tests/taskserver.nix) this
branch adds and it fails because of the changes introduced by the
closure-size branch, so we need to do additional work on base of this.