Custom Systemprompt extension should extend default prompt instead of replacing

[surt91]

The custom systemprompt extension currently replaces the default systemprompt. Since the default prompt contains some generally useful instructions (e.g. about markdown and latex support), most user will probably want to extend the default instead of replacing it.

However, there might be cases, where e.g. the current date should not be known to the model. Therefore a checkbox "Replace default systemprompt" in the custom systemprompt extension should be offered.