Disable xfce4 screensaver rather than light-locker (add user docs)

[JimMadge]

✅ Checklist

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🚦 Depends on

⤴️ Summary

Adds an Ansible task to disable xfce4-screensaver.
This appears to not work due to a bug in xfce < 4.18.

Adds user documentation to disable screen locking if they find it a pain.

🌂 Related issues

Related to #2101

🔬 Tests

[JimMadge]

Testing in an SRE.
The file is written as expected

jim.madge:autostart$ tail xfce4-screensaver.desktop
Exec=xfce4-screensaver
OnlyShowIn=XFCE;
NoDisplay=true
X-XFCE-Autostart-Phase=Application
X-XFCE-Autostart-Notify=true
X-XFCE-Bugzilla-Bugzilla=XFCE
X-XFCE-Bugzilla-Product=xfce4-screensaver
X-XFCE-Bugzilla-Component=general
X-XFCE-Bugzilla-Version=4.16.0
Hidden=true

[JimMadge]

Started a new xfce session by logging out then in again.
Waiting to see if the screen locks after idling.

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[JimMadge]

Still locking after ~10 minutes.

[JimMadge]

There is this bug report which suggests there was an issue in xfce4-session<4.18 and we have xfce4-session==4.16.

[JimMadge]

Uninstalling xfce4-screensaver seems to work and doesn't break the xfce installation.
It does mean that users can't intentionally lock the xrdp session.

I think we are comfortable with this though because users will always be on a client machine that they can lock.

Actually, that is a bad idea. The screen blanks and you are unable to unlock it 😱.
@jemrobinson I think it might be best to revert the change and accept this.
Normal users should be able to disable screen locking after idle in the Xfce4 settings or by running xfce4-screensaver-preferences. We could add that to the user guide?

[jemrobinson]

Can you add it to the user guide as part of this PR? Ideally, after merging #2130.

[JimMadge]

Yep, just testing that it works now.

[jemrobinson]

LGTM - a couple of suggestions.