Update and refactor random password generator

common/djangoapps/student/management/commands/manage_user.py

...n/djangoapps/student/management/tests/test_manage_user.py

common/djangoapps/student/views/login.py

common/djangoapps/student/views/management.py

common/djangoapps/third_party_auth/pipeline.py

common/djangoapps/third_party_auth/tests/test_pipeline.py

lms/djangoapps/dashboard/sysadmin.py

openedx/core/djangoapps/external_auth/views.py

...edx/core/djangoapps/user_api/accounts/tests/test_utils.py

openedx/core/djangoapps/user_api/accounts/utils.py