Prevent Linked repository from deleting in the UI until all the plans that use this no longer uses

Problem Definition

When we delete a linked repository from the UI, it throws a warning message listing the number of plans that are referenced just before it is deleted. Once deleted, the repository will not appear in the Linked Repositories section. However, all the plans where this linked repository is associated will continue to work and one may have to manually delete them from the respective plan configuration.

Suggested Solution

The linked repository shouldn't be deleted from the UI until all the plans that uses this repository is deleted/replaced with some other linked repository.

Why this is important

imagine a list of 10 plans. by the time the user has cleaned up the first one s/he no longer remembes which were the other 9 and there is no way to find out which they are anymore except by doing database queries.

This creates inconsistency and introduces confusion when we create a new linked repository in the same name.