Listen to websocket event with next epoch information when available

[ylembachar]

Right now, we are refetching the data on the slot overview page at the beginning of every epoch to get the current epoch data as well as the next one in the slot progression component. The next data is not always available, and should be broadcast using the websocket event, however, at the moment the websocket event does always not contain the data for the next epoch.

Change to listening to the websocket event when the data is available.