[plugins/scoreboard] Allow public scoreboard view

[anthonygego]

Is your feature request related to a problem? Please describe.
Scoreboard is not available when using the course via LTI, though it does not require write access to the database. By default, LTI courses that send scores back to the host platform are not accessible to the students to prevent them from submitting from INGInious only. Indded, LTI sessions cannot be open from INGInious.

Describe the solution you'd like
The scoreboard only could be accessible to registered course users. Request by @tbarbette .