Add a option for a default notice instead of the HEADER if not specified otherwise

I want to keep the HEADER as clean and fast as possible (as you might have guessed from the other two issues^^). In order to achieve this, a default notice would be great which is customizable.

1. Admin has the option to choose the default notice

if activated the app will display a custom note (max one line) to the user if no dedicated HEADER.md file is available in this folder
a Tag {README.md} is available to automatically set the link which looks like that: <a href="#app-content-headerMD">README.md</a>
styling: a H4 heading would be well proportioned

2. Admin has the option to choose a background-color

same setting as for the README.md
give the option to choose a color if not to complicated

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