Improve editor experience

[matuella]

Currently, the editor doesn't allow specific-code highlight, do not allow uploading images and has an overall poor text-editing experience, due to some implementation decisions in flutter-quill.

We should look into these issues and, if they prove to be big enough changes, probably fork the flutter-quill and start from there. Creating a brand new project for this is also a valid possibility, given that the code in flutter-quill do not contain any unit test and has almost no documentation, making it hard to update things there.