System warning

[solth]

This Pull Request adds the option for admins to set system messages and send nofitications to all users about, for example to inform them about upcoming maintenance work or server shutdowns.

Two methods are added to facilitate these system notications:

1. setting a "Static system message" that is displayed to all users on all pages when it is activated, comparable to the old functionality of Kitodo 2 (see screenshots 1 & 2)
2. sending a "Broadcast" to all users currently logged into the system via websockets. This results in a modal popup dialog shown to the users which they must acknowledge by clicking on "Ok". The admin who broadcasted the message can see which users acknowledged the message on the system message page. (see screenshot 3 & 4).

Screenshot 1: write a static system message

Screenshot 2: static system message is displayed to all users

Screenshot 3: write a broadcasting message to all users and monitor which users acknowledge the message:

Screenshot 4: broadcasting message displayed to users in modal dialog to acknowledge:

Fixes #3586

[Kathrin-Huber]

The loginpage is broken (no orange footer, login without margin), because it is taken out of "base.xhtml"

[Kathrin-Huber]

The new authority is not translated