Resolve "Manually enrolling students: button broken"

What does this mr do?

Solves enrol button for manually enrolling students into an exam slot by making it clickable again.

Actions taken to fix bug

Every time a slot is selected, make the current z-index 1 higher than all the other slots such that the selection form is clickable.

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #315 (closed)