Allow only a specific list of students to check in for some trainings

[ArtemSBulgakov]

Describe the feature

Currently admin can select groups which are allowed to attend some sport group (e.g. general medical group / special 1 / special 2). But some trainings allow only a specific set of students to attend. So, we need to implement a feature to specify some list of students, and only these students will have the right to attend the sport group.

Suggested solution

• Add field 'allowed students' to the group. This field will store ids of students.
• Add checks in check-in method, so that these students are allowed to check in (if there are enough places left).

Add the field to the admin page below 'allowed gender' field:

Additional context

No response