Implementation deletion concept

Description of changes

implemented deletion concept in media service: media records update their content IDs when content is deleted

Please describe the test strategy you followed.

Manually tested by creating content, linking media records to the content and then deleting the content via course/chapter deletion.

[X] My code follows the coding guidelines of this project
[X] I have performed a self-review of my code
[X] I have commented my code, particularly in hard-to-understand areas
[X] My code fulfills all acceptance criteria
[ ] I have added tests that prove my fix is effective or that my feature works
[X] New and existing unit tests pass locally with my changes
[X] The test coverage (line and branch) is reasonably high, especially on Service classes and other classes with complex logic
[X] I have made corresponding changes to the documentation
[X] I have added explanation of architectural decision and rationales to wiki/adr
[X] I have updated the changes in the ticket description

[ ] The code works and does not throw errors
[ ] The code is easy to understand and there are no confusing parts
[ ] The code follows the coding guidelines of this project
[ ] The code change accomplishes what it is supposed to do
[ ] I cannot think of any use case in which the code does not behave as intended
[ ] The added and existing tests reasonably cover the code change
[ ] I cannot think of any test cases, input or edge cases that should be tested in addition
[ ] Description of the change is included in the documentation