Use DynamicUser instead of a static kolibri user

[dylanmccall]

As discussed in pull request #1, this removes the static kolibri system user, and instead uses systemd's DynamicUser= option. That allows us to greatly simplify how this component is implemented.

Unfortunately, dbus policy files don't work with dynamic users (systemd/systemd#9503), so the following policy breaks:

  <policy user="kolibri">
    <allow own="org.learningequality.Kolibri.Daemon" />
  </policy>

At the moment, I am changing the policy to allow any user to own the Kolibri daemon's name on the system bus, but that is not ideal. I think we would need a better solution to that problem before merging this.

[dbnicholson]

Note that you almost definitely need to keep the predefined system kolibri user even if you do this because DynamicUser doesn't deal with our persistent user scheme correctly across ostree upgrades IIRC. I can dig up the details later, but I know we went through this exercise before.