CMS-4585 User Manager - New Group and Role: validation

Disabled the Principal creation on a new wizard initialization.
Added validation for the name filed in header, since other fields are
optional.
Disabled the name input for edit mode, since the key won't change in
future.
Fixed the `PrincipalKey.toPath()` method,.
Removed `UserTreeGridItem.getDataPath()`.
Merge pull request #1066 from enonic/CMS-4585

CMS-4585 User Manager - New Group and Role: validation