Add rudimentary copywriter functionality, implement assistant metatask wrapper

[MB-Finski]

This pr implements rudimentary copywriter functionality for assistant.

• Uses a free prompt provider in the background
• Implements an assistant meta-task wrapper to differentiate between plain free prompt tasks and custom internal task types
• Generalizes the wrapper for all current task modalities to unify the notification system.
• Implements a text file picker for all assistant task types in the front-end. Currently supports ODT, DOC, DOCX, RTF, MD and plain text by using PhpOffice/PhpWord and parsedown to parse the files at the backend.

Known issues:

• Some psalm issues still remain
• The UI is not ideal but possibly "enough for an MVP".

[MB-Finski]

Oh, and task "modality" has not yet been renamed: I'm open for suggestions!

[julien-nc]

Suggestions:

• family
• category
• group
• taskOcpNamespace
• feature

[julien-nc]

I feel like there are some files missing?

@marcelklehr Which ones?

[marcelklehr]

I feel like there are some files missing?

@marcelklehr Which ones?

Nevermind, may have been a Github glitch