Admin Console error when editing group with non-local members

From https://github.com/igniterealtime/Openfire/pull/1708#issue-490258070

There is a usage in the form of webManager.userManager.getUser(member) in the group-edit.jsp file. The member is of type JID, but it is converted to string when called. And it comes to getUser method as "username@example.com". This is "java.lang.IllegalArgumentException: The input 'username@example.com' is not a valid JID node part: Contains prohibited code points." causing error.