nixos/lightdm: add `greeters.mobile` config option

i based this on the neighboring lightdm-greeters/mini.nix module.

lightdm-mobile-greeter doesn't have a lot of configuration options.
it grabs the default user to login as from lightdm, along with which DE
it should launch. so no further configuration should be needed aside
from enabling `services.xserver.displayManager.lightdm.enable` and
either setting `services.xserver.displayManager.defaultSession` to the
appropriate session or explicitly defining a seat like:

```nix
services.xserver.displayManager.lightdm.extraSeatDefaults = ''
  user-session = phosh
'';
```

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