Mark Job as taken, change status for all associated applications

### Describe the solution you'd like
If an employer marks a job as filled, then all existing applications for this job should be changed in status. At the moment, unfortunately, nothing is happening that is badly solved and should be improved. The applicants do not even realise when a job is taken.

Start by tracing the employer action that marks a job as filled and how associated applications store and display their status. Confirm the desired status transition and notification or visibility behavior, then verify that every existing application for the filled job reflects the new status.

Written by the indexing model from the issue text.