User: `default` is special too
Cleanup: refactored special users
Cleanup: renamed method/attribute to align semantics
User manager: added method to hide permission-related users
User admin: hide permission-related users
WTF this logic
UserSet: removed unnecessary arguments
Textarea: also allow auto-focusing
Fields: also allow setting as read only
User: defined what a meta user is
User admin: username is read only for meta users
User admin: hide irrelevant information from meta users
User: protect meta users from being deleted