Missing user on lightdm login screen

Missing user on lightdm login screen

We identified 3 possible "common" causes:

1) If no user shows up at all, check for `greeter-hide-users=false` in `/etc/lightdm/lightdm.conf.d/*` , if missing create it.
   See also: https://wiki.debian.org/LightDM#Enable_user_list
   
2) If only some of the users shows up, or step 1 is already correct. Check for MIN_UID/MAX_UID settings.
   Most likely locations: 
   - /etc/lightdm/users.conf   --> `minimum-uid`
   - /etc/login.defs
   
3) Make sure there **exists** an entry for your account in accountservices config folder `/var/lib/AccountsService/users/<username>`;
and that the `SystemAccount` options is set to `false`.
Minimal file for user `AAA`:

```
[User]
Language=en
FormatsLocale=en_US.UTF-8
XSession=cinnamon
SystemAccount=false
```
   
   
It's worth mentioning that lightdm appears to fetch information about users and login configuration from different locations depending on configuration/service availability.
   - /etc/{passwd,shadow,group}
   - /etc/login.defs
   - accountservices