Rename User aggregate root to Student

## Summary
Currently in domain layer aggregate responsible for managing users uses `User` naming (folder which contains everything related to aggregate is `Users` and aggregate root is `User`). But `Student` naming gives better semantics in code (we are concerned only about student users). Therefore refactoring is needed.

## [Acceptance criteria](https://www.atlassian.com/work-management/project-management/acceptance-criteria)
- [ ] Rename diretories, files from User/Users to Student/Students
- [ ] make sure that all tests, variables, comments, etc use new naming
- [ ] add new migration as db table is named `users` (should it be singular or plural?)

## Notes
important context