Make sure we can reinstall a brand new Jenkins if we ever need to

The following items are currently manually configured in our Jenkins instance and are not properly documented:

• "Configure System" → "Jenkins URL": https://jenkins.tails.boum.org
• "Configure Global Security" → "TCP port for agents inbound connection" → "Fixed": 42585
• For preferred nodes: "Configure" → "Node properties" → "Preference of Node" → "Preference Score": 10

Possibilities:

• SCM Sync Configuration plugin: deprecated, but we already used it and it might be easy to continue using (see #7029 (closed)).
• Configuration as Code plugin (doc): maybe more maintainable in long term (as it's not deprecated as the above), but not sure how much work it'd be to port and if it pays off.
• Manual documentation: depends on admins remembering to do it everytime.
• Rely on configurations being saved in backups.