Schedule enhancements

[Markusplay]

• 2) Remove data storage of the selected group and pairs in local storage
• 3) Validate the selected group and course drop-downs in import
• 4) If the backend did not return an error - immediately show the toast of successful import. Change the text, for example, to "request sent successfully" - deprecated.
  New
  Show loading state to show user that importing is in progress
• 5) If user is authorized, put his group to schedule dropdown, else don't insert