Fix/121 user list responsiveness

This MR addresses the issue where the user list wasn't responsive on smaller screens (look at the user list within a chat session):

• Toggle button is only displayed on short screen to display the user list
• the user list is now adapted for different screen sizes

Remark: There are other pages/components in the app that are not fully responsive yet.

frontend/src/lang/en.json

@@ -144,7 +144,8 @@
 			"participants": "Participants",
 			"programed": "Scheduled",
 			"status": "Status",
-			"topics": "Topics"
+			"topics": "Topics",
+			"toggle": "Participants"
 		}
 	},
 	"button": {