[ENHANCEMENT]: Remove modal for subtasks of supertask

[novasam23]

Description

The modal to be removed can be found as follows:

1. Got to http://localhost:4200/#/tasks/supertasks
2. Click on the three-dot menu on right of a supertask
3. Select "Edit Subtasks"

The modal is implemented by the ModalPretasksComponent class.

Also required for this change is to make the following columns editable on the edit page of a supertask (e.g. http://localhost:4200/#/tasks/30/edit), see PretasksTableComponent:

• Priority
• Max. Agents