Restrict to only allow 100 students in a section

[Fixes #72753188]
As title, for the test case, I am currently only testing it in the Logic part.
Should I test it in UI and Db part as creating 100 students for a course may slow down the test ?
Rephrase as: The team t1 is in multiple sections. The team ID should be unique across the entire course and a team cannot be spread across multiple sections.