Customise xrdp

[jemrobinson]

:white_check_mark: Checklist

• [x] You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• [x] You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• [x] Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).

:vertical_traffic_light: Depends on

n/a

:arrow_heading_up: Summary

• Customise xrdp login screen
• Apply some xrdp-sesman settings to block root login and kill disconnected sessions

:closed_umbrella: Related issues

Closes #2086

:microscope: Tests

Tested on fresh SRE deployment

[github-actions[bot]]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/commands
shm.py					66, 68, 70, 72-99
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