What problem are you trying to solve with Submitty
Currently, when a chat session is ended there is no way for a student to see a chat history. However, if the chat session were to stay open so that a student could see that history they would be able to send messages, which isn't optimal. An option for chats should be added which makes ending sessions keep the chat page viewable but prevents users from sending messages.

Describe the way you'd like to solve this problem
A checkbox should be added to EditChatroomForm and CreateChatroomForm which enables this new feature, and still shows chats in the list after a chat session is ended. These chats can be entered, but no message entry is shown. This would require adding a flag for chats which have been enabled, as we don't want to show empty upcoming chats. Either that, or we can check for message count being above 0 for read only chats before showing them in the list.

Describe any potential alternatives you'd tried to solve the problem
An alternative implementation, which I haven't tried, is to instead have a separate checkbox for each chat on the list of chats labelled "show read only when closed" or something, which then shows the chat even when a session is closed.

Additional context
N/A