I18n fix by Tsinghua

I fixed some i18n bugs which are obvious and easiest to fix. Most of them are in HTML templates.
Including following fixes:
1.  i18n:
cms/djangoapps/contentstore/views/assets.py
cms/static/client_templates/course_grade_policy.html
cms/templates/asset_index.html
cms/templates/widgets/units.html
common/djangoapps/student/views.py
lms/templates/dashboard.html
lms/templates/forgot_password_modal.html
lms/templates/login.html
2.  i18n error:
cms/templates/index.html
3.  over i18n:
lms/templates/courseware/course_about.html
4.  using wrong template:
lms/templates/registration/password_reset_complete.html
lms/templates/registration/password_reset_confirm.html