Revamp Delete Repo Content Workflow

[giovanni-guidini]

This epic issue captures the tasks related to fixing the current Delete Repo Content workflow.

Overview

From the user's perspective we hope to replace the message on the current confirmation modal to make more evident that the process can take some time and how the user will be notified once it's done.

We also want to change the toast message confirming that the task has been kicked off.

On the feedback side we will send a transactional email for the repo's admins confirming that the repo deletion is completed successfully of failed for some reason and they should reach out to support.

On the backend side we will make sure the task is functioning properly and improve its performance as much as possible. And trigger the email delivery.

Success Criteria

The success criteria is that the following flow can be executed:

A user can request deletion of repository content fro which the user is an admin of.
After some (reasonable, meaning at most 1 day) time the user will receive confirmation via email that the repo contents have been deleted.
After receiving the email the user can navigate to the Codecov UI and see that the contents of the repo have indeed been deleted.

Tasks - platform

Preview Give feedback

1. Make sure FlushRepoTask deletes all the entities related to Repository #1134
   0 of 4
2. Parallelize deletions of unrelated objects #1135
3. Create temp Owner to assign Repo to in order to delete from UI instantly #3225
   

Tasks - applications

Preview Give feedback

1. UPDATEME - Change wording on repo content deletion confirmation modal
2. UPDATEME - Change color and text of the toast message confirming repo content deletion request

[Swatinem]

Code for both the Repository and Owner deletion was updated, and should be working now.
Both of that is hooked up within the django admin for customer support folks to trigger it.

The self-serve repository deletion should also be working now.
For the self-serve owner deletion, there is a couple more PRs that would need landing:

• Unify and slightly reword account deletion gazebo#3742
• Reintroduce the "delete account" action gazebo#3745

Thus, I would call this epic done.