Validate exam grade submission availability

This MR closes #324 (closed) by performing an on load exam grade submission availability check and showing the results to the instructor.

• If the course's department has the gradeSubmissionPeriodEnd or gradeSubmissionPeriodStart defined, those dates will be shown, for example:

• If the grade submission period end is defined and the current date is greater than it, the instructor cannot upload grades and thus they see:

• If the grade submission period start is defined and the current date is less than it, the instructor cannot upload grades and thus they see:

• If the exam is closed, there is no change in the UI. The already shown information is sufficient.

Note: The above messages-the ones that refer to the validation result-are sent localized directly from the server.