Improve warnings for unsaved paragraphs and other form changes

Problem/Motivation

Paragraphs can be changed by editing their fields or even the referenced entity. Since that is perceived as a separate operation because there is an inline form that looks as if you're finished after editing, but to really save your edits you need to also save the parent form (article). Paragraphs module shows a warning inline with the field that is not very visible and if you edit another paragraph the warning for the fist paragraph vanishes and a new warning appears at the newly saved one's reference field.
There is an issue for paragraphs module that partly deals with that regarding the display of the message: #2738715: Improve Paragraph collapsed/closed summary
However the whole system of checking if the content form needs to be saved needs to be improved, to warn and notify the user if she/he might loose data.

Proposed resolution

Improve the visual presentation of these warnings in the form. Create a frontend behaviour that tracks changes in the form for both paragraphs and other fields and notifies the user when she/he wants to navigate away from the form without saving.

Remaining tasks

1. Create wireframes to show how to improve that
2. Create a design for the backend theme thunder_admin
3. Test the paragraphs patch
4. Implement a proof of concept in code
5. See if this can be a patch for paragraphs or needs to be implemented separately