Added a REST URI to retrieve the number of checked-in visitors... - #308

... in a room at a given moment.

Related Issue

• Feature Request: API for room occupancy #307

Checklist

• Is the code formatted correctly?
• Unnecessary comments removed?
• Printout statements removed?
• If you made backend changes: Did all tests pass? Do you need to adjust some tests or write new ones?
• Is your code easy to understand or do you need to insert some comments?
• Explain the issue for the reviewer and your steps to solve the issue => makes it a lot more easy for the reviewer
• Label your pull request (frontend/ backend/ testing/ styling/ sql/...)
• Is the open source message included at the very beginning of every source code file?
• Are all imports organized, i.e. are the imports up to date and have all package imports been removed?