UX and UI improvements on nuxeo-document-permissions

Nuxeo-document-permissions element has several UX and UI problems:

1. The term "Right" is a little bit confusing. The general pattern to define it is "Permission";
2. On adding a new permission, the button "Create and add another" is centered, which constitutes a non existing pattern;
3. When a button is disabled, user has no visual cue of it;
4. When we are editing an existing permission, we loose context on which user we are applying the changes;
5. On editing a permission, it is not clear what fields are required;
6. On deleting a permission, the table inside the dialog is inconsistent and the buttons are not properly aligned.
7. The "Share with external user" popup is inconsistent with the "Permissions defined locally".