Resolve "Lab page should also have info on totals"

What does this mr do?

Add an indicator to show how many slots are taken in total for this lab.

Screenshots

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #516 (closed)