#119 Improvements of teachers API and features around it

* Add external keys to departments table and update inserting teacher API
* Add ability to set department by External ID
* Refactor error handling in teacher api
* Update description of the method
* Add functions for creating activated (from 1C) teachers
* Add ability to fire teachers
* Add status and INILA (Insurance Number) to teacher's table
* Extract stored migrations about departments and jobs to dedicated file and rename them