Check Availability of name in settings

[oduvan]

As there is an options of using content settings in combine with the settings we need a validation of the name intersection when creating a new setting.

CONTENT_SETTINGS_DJANGO_COMBINE=False

by default the setting is False, means separate content_settings is using in code.

If True - the combined settings will be used, and all validation should be done.

Combined context can be used only in case of CONTENT_SETTINGS_DJANGO_COMBINE=True