Remove double quotes in generated sql query in assignment_context_scope, which fails in mysql.

Description:
visit: /api/v1/calendar_events?type=assignment&context_codes...&per_page=50

The double quotes in table and column names gives a mysql syntax error:

(Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '."context_code" IN (','