Remove flagged exercises in accordance with design doc

[flacial]

Description

The current implementation for removing flagged exercises differs from the design doc. According to the design doc (#1192), a removed exercise should not be visible to students, but it should not be completely removed from the database.

We should update the implementation to match the design doc. This will ensure that the app behaves as intended and meets the requirements outlined in the design doc.

Steps to reproduce:

1. Remove an exercise with deleteExercise as inappropriate
2. Observe that the exercise is removed from the database instead of just being hidden from students

Expected behavior:

Removed exercises should be hidden from students, but not completely removed from the database

Actual behavior:

Removed exercises are completely removed from the database

Solution

Update the implementation for removing flagged exercises to match the design doc. This will involve modifying the code so that flagged exercises are hidden from students, but not completely removed from the database.

Plan

Create a resolver that specifically applies the solution, create the fields bool named removed, user named removedBy, and date named removedAt in the Exercise schema.

[flacial]

Closed in #2668