Allow to select only tasks that are currently open to students

With an additionnal button. This could even be extended with a nice dropdown, that allow to select all tasks with a certain tag.

Example of dropdown content:
- Select all tasks with tag bla
- Select all tasks with tag blu
- Select all tasks with tag blo
- Select all tasks open to students
- Select all tasks closed to students
- ...

Start by locating the existing task-selection interface and the code that determines whether a task is open to students. Review how task tags and student visibility are represented before choosing between an additional button and a dropdown. Done means users can select all currently open tasks, with the proposed tag-based choices supported if included.

索引モデルが issue の本文から書いたものです。