This PR introduces a proper `action` to the topic timeline `back-button` which will fix the button not being clickable (or functional) as well as removing a duplicate setting of `this.lastReadTop` which was causing odd positionings of the button.
This is very difficult to test due to the fact you have to manage the "read history" for a user to have the back button populate. We will have to move forward without one (as we did in the last version of the timeline 😅) for now.