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.

nixos/modules/misc/ids.nix

@@ -261,6 +261,7 @@
       syncthing = 237;
       mfi = 238;
       caddy = 239;
+      taskd = 240;
 
       # When adding a uid, make sure it doesn't match an existing gid. And don't use uids above 399!
 
@@ -493,6 +494,7 @@
       syncthing = 237;
       #mfi = 238; # unused
       caddy = 239;
+      taskd = 240;
 
       # When adding a gid, make sure it doesn't match an existing
       # uid. Users and groups with the same name should have equal