Dark mode: common settings

[chibenwa]

• Define a JMAP setting for dark mode
• Include it in read only settings somehow (like language)
• RabbitMQ wiring for common settings

[chibenwa]

We likely need a spec for this too CF https://www.notion.so/linagora/ADR-015-Common-settings-V-2-0-20062718bad18072afb5c7d40ae26782 has no ref for dark theme