FE: Complete transaction button

[esteban-gs]

Add a "Complete Transaction" button if it doesn't already exist in the transaction messages view. The button should have an action that makes a PUT request to the backend that changes the Transaction's status to Fullfilled.

In the UI, there should be a UI rule in place that makes all input fields read-only when a transaction has the Fulfilled status, meaning no more real-time conversations can take place. However, the conversation can still be visible.