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/doc/manual/default.nix

@@ -57,6 +57,7 @@ let
       chmod -R u+w .
       cp ${../../modules/services/databases/postgresql.xml} configuration/postgresql.xml
       cp ${../../modules/services/misc/gitlab.xml} configuration/gitlab.xml
+      cp ${../../modules/services/misc/taskserver/doc.xml} configuration/taskserver.xml
       cp ${../../modules/security/acme.xml} configuration/acme.xml
       cp ${../../modules/i18n/input-method/default.xml} configuration/input-methods.xml
       ln -s ${optionsDocBook} options-db.xml