Migrate injections to prepare for new Exercise Group format

[elbotho]

When removing GroupedExercise after #252 is done we would break > 700 injections.

To avoid this we need a migration that changes the state of all injections that target a GroupedExercise.

Instead of {grouped-exercise-uuid} or /{grouped-exercise-uuid} it should be:
{exercise-group-uuid}#{grouped-exercise-uuid} than we can restore the existing functionallity in the frontend.

Optional bonus points:

• remove injections with empty strings
• remove injections with non-existing uuids

Potentially helpful:
https://github.com/serlo/db-migrations/blob/main/src/utils/edtr-io.ts

status

• deploy Deployment frontend#3429
• run migrations:
  • Update uuid links in injections: feat: Update uuid links in injections #271
  • Migrate injections of grouped exercises: feat: Migrate injections of grouped exercises #275