user-accounts: Add a launcher for parental controls app

malcontent ships malcontent-control[1] which is the user interface
to interact with various parental-control settings. Add a launcher
as per the design mockups[2] to launch malcontent-control.

[1]: https://gitlab.freedesktop.org/pwithnall/malcontent/-/blob/master/README.md
[2]: https://gitlab.gnome.org/Teams/Design/settings-mockups/blob/master/users/users.png
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