Make sure `FlushRepoTask` deletes all the entities related to `Repository`

[giovanni-guidini]

After reviewing the FlushRepoTask it's clear that some objects that should be deleted are not being cleaned up. This task consists of making sure everything is properly deleted.

You'll notice that the task deletes things table by table instead of relying on cascades of the database. This is done to reduce the number of records deleted in each transaction, reducing the total time of the query. We've had issues with queries running too long before, so we prefer to have many queries that run faster.

This task list might not be complete, but includes known objects that need to be deleted within the task. Check the image below (pretty big) for the relationship of different tables that are related to repos.

Objects to add to FlushRepoTask

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1. Delete raw uploads associated with Upload objects;
2. Delete RepositoryToken objects;
3. Delete CompareComponent objects;
4. Delete LabelAnalysisProcessingError objects;

[Swatinem]

The revamped FlushRepo code (codecov/worker#973) is resolving all relations using django metadata, so does solve this issue.