Added support for multilanguage text in the instructions for enrollments

Migration script
```java
package pt.ist.scripts;

import java.util.Set;

import net.sourceforge.fenixedu.domain.EnrolmentInstructions;

import org.fenixedu.bennu.core.domain.Bennu;
import org.fenixedu.bennu.scheduler.custom.CustomTask;
import org.fenixedu.commons.i18n.I18N;
import org.fenixedu.commons.i18n.LocalizedString;

public class EnrollmentInstructionsMigration extends CustomTask {
    @Override
    public void runTask() throws Exception {
        Set<EnrolmentInstructions> instructions = Bennu.getInstance().getEnrolmentInstructionsSet();
        for (EnrolmentInstructions instruction : instructions) {
            LocalizedString newString = new LocalizedString(I18N.getLocale(), instruction.getInstructions());
            instruction.setInstructions(newString.json().toString());
        }
    }
}
```

Note: Run this script before apply the changes made in this commit

close #581