Resolve "Editing of capacity does not work unless focus is explictly lost"

Summary

Before, editing a capacity of a slot when creating/editing a slotted lab required the user to explicitly lose focus from the capacity textbox for the new capacity to be saved. Now the capacity gets saved as soon as you enter a number. There are also checks for NaN and negative capacities.

Checklist

All items on this checklist should be checked.

• I have added a changelog entry.
• I have added a test that catches the original bug behaviour.
• I adhere to the style guide.

How to review

1. Log in as somebody allowed to create a new slotted lab
2. Choose or make a course
3. Start creating a slotted lab, and make some slots
4. Before savng the lab, choose a slot and edit the capacity to see that it changes immediately
5. Try to put in text or a negative number to see that they do not get saved as a valid capacity
6. Save the lab
7. Edit the slot capacity from session overview to see that it also works there

Issues and tasks closed

Closes #766 (closed)