Fix bugs related to SQLAlchemy and database operations

[SuyashD95]

• Add the correct name of child Model classes used to declare relationships in the parent Model class.
• Resolve NoReferencedTableError that occurs due to issues in making relations via Foreign Keys.
• Resolve a bug that stops a new room from being created.
• Remove timestamp field from messages table to solve issues resulting from working with a DateTime object.
• Resolve a bug that stops a new message from being created.
• Resolve bug which prevents a MessageModel object from backreferencing the associated room.