PC-30: Users management in ActiveAdmin

[IvanRuskevych]

Hello Team, please review this PR.

1. Add a tab for user management to the ActiveAdmin dashboard, allowing administrators to view, create, edit, and delete users.
2. Fixed Heroku deployment error
3. Fixed line separators endings to LF
4. Fixed the issue by adding gem 'sass-rails', '~> 5' and removing gem 'sass'

[Maryna-Harasko]

The check request.params.dig(:user, :password).present? in permit_params is unnecessary since ActiveAdmin does not send empty fields — it can be removed. Make sure that password updates work correctly when updating a user.
permit_params :name, :email, :password, :password_confirmation

[IvanRuskevych]

If we use permit_params :name, :email, :password, :password_confirmation, then entering a password becomes mandatory, when updating a user, which does not meet the requirements "The password field is optional and will only be updated if filled." (PC-33).
Therefore, the following approach is applied: params += [:password, :password_confirmation] if request.params.dig(:user, :password).present?
This ensures that the :password and :password_confirmation parameters are added only if the password field is not empty.