Allow notice file generation from OpossumUI

[benedikt-richter]

As a user of OpossumUI I want to be able to create a notice document in an easy integrated manner.

To achieve this goal an established notice generator package might be used to generate a notice from within the app.

[vasily-pozdnyakov]

Maybe not really a notice document, but at least some human-readable file.
Also think about preview in the Opossum UI.

[mstykow]

Here we need to define a bit more clearly what we mean by a "notice file". We can compare with what we generate in closed-source code and take a similar approach here. Also note, that we should generate that file in both txt and HTML.