Backward compatible in terms of literals already translated containing "RELATE" as context in templates.

overriding.

	course/views.py
	locale/de/LC_MESSAGES/django.po
	locale/zh_CN/LC_MESSAGES/django.po

remove help_text, which duplicate validation error. update validation error string, which is not complete.

Use django styled username validation rule. Update some literal in auth.py with django default so that need for translation for them. Provide translations for new literals.