Implement bulk download for missions

@raymondtangsc , I think there is a way to remove this line by modifying the routes file, do you have any ideas?
@mission = @course.missions.find(params[:assessment_mission_id])

Now it works, but think there must be a better way.
Style/CaseIndentation: Indent `when` as deep as `case`.<br>Style/StringLiterals: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.