Issue #86 Add a callback to filter sections from course format (#92)

* Issue #86 Add a callback to filter sections from course format
* Issue #86 Use value, not key, for massaction_restricted_sections callback
* Issue #86 Check if the source section is restricted
* Issue #86 Fix phpunit error
* Issue #86 Check if the dropbox exists before filtering
* Issue #86 Throw exception when selected section is restricted