3) Bug fix: Sending messages

[SV62]

We have some code in place for sending messages in existing chats, but it is not working as expected - we are unable to properly send messages, and the messages are not displayed in the correct order on page load. Additionally, there is a security issue related to sending messages where a user can send messages to any conversation, not just their own conversations. Please investigate and resolve these bugs.
Note: Changes for this ticket should not involve changing the code for sockets, which will be addressed in ticket #4