Admin needs to be able to send email reminders to Assessors

[Rebecca-mcknight1991]

Is your feature request related to a problem? Please describe.
As an admin I need to be able to send reminder emails to Assessors where the Independent Assessment is outstanding.

Describe the solution you'd like
Issue #319 gives a solution for monitoring returns. It is suggested there would be a way from this table to:

1. Filter out assessments which are completed only leaving the outstanding ones
2. Select all the candidates remaining
3. Send a bulk reminder email to their assessors
4. Alternatively click send reminder on a specific candidate

See mock up below (purple buttons)

Describe alternatives you've considered
Google drive but there is not an easy way to monitor and send reminders from here.

Additional context
Nothing else.

[warrensearle]

The following cloud function should be used for sending a request to assessors:

sendAssessmentReminders

An object containing the following data should be provided as a single parameter:

• exerciseId (required) ID of exercise
• assessmentId (optional) ID of an assessment
• assessmentIds (optional) array of assessment IDs

Example: send a reminder for a specific assessment

sendAssessmentReminders({
  exerciseId: 'exercise123',
  assessmentId: 'assessment56',
})

Example: send assessment reminder for a selection of assessments

sendAssessmentReminders({
  exerciseId: 'exercise123',
  assessmentIds: ['assessment1', 'assessment5', 'assessment56'],
})

Example: send assessment reminders for all assessments in an exercise

sendAssessmentReminders({
  exerciseId: 'exercise123',
})