-- Messages shown to all users as soon as they visit the site/log in (i.e.: "System is going down for maintenance next sunday")
-- Only administrators (of any school) should be able to create these via a web-interface
SystemMessage
    from                UTCTime Maybe -- Message is not shown before this date has passed (never shown, if null)
    to                  UTCTime Maybe -- Message is shown until this date has passed (shown forever, if null)
    authenticatedOnly   Bool -- Show message to all users upon visiting the site or only upon login?
    severity            MessageStatus -- Success, Warning, Error, Info, ...
    defaultLanguage     Lang -- Language of @content@ and @summary@
    content             Html -- Detailed message shown when clicking on the @summary@-popup or when no @summary@ is specified
    summary             Html Maybe
SystemMessageTranslation -- Translation of a @SystemMessage@ into another language; which language to choose is determined by user-sent HTTP-headers
    message             SystemMessageId
    language            Lang
    content             Html
    summary             Html Maybe
    UniqueSystemMessageTranslation message language
