Allow masquerading as a specific user different from the logged in user.

common/lib/xmodule/xmodule/abtest_module.py

common/lib/xmodule/xmodule/capa_module.py

common/lib/xmodule/xmodule/conditional_module.py

common/lib/xmodule/xmodule/foldit_module.py

common/lib/xmodule/xmodule/html_module.py

common/lib/xmodule/xmodule/library_content_module.py

common/lib/xmodule/xmodule/randomize_module.py

common/lib/xmodule/xmodule/seq_module.py

common/lib/xmodule/xmodule/split_test_module.py

common/lib/xmodule/xmodule/vertical_block.py

common/lib/xmodule/xmodule/video_module/video_module.py

common/lib/xmodule/xmodule/x_module.py

lms/djangoapps/courseware/masquerade.py

lms/djangoapps/courseware/model_data.py

lms/djangoapps/courseware/module_render.py

lms/djangoapps/courseware/tests/test_entrance_exam.py

lms/djangoapps/courseware/tests/test_masquerade.py

lms/djangoapps/courseware/tests/test_module_render.py

lms/djangoapps/courseware/tests/test_submitting_problems.py

lms/djangoapps/courseware/tests/test_views.py