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: You are trying enroll more than 100 students in section S1. To avoid performance problems, please do not enroll more than 100 students in a single section.