Add visibly hidden information about the time and room when the user is not shown the "choose slot" dialogs

[joyheron]

When the user wants to navigate through the table when there are currently no "choose slot" actions shown, we should add visually hidden span elements to the <td> elements to let the user know which slot/time they are currently in. This information is already included in the "choose slot" actions, so it isn't needed there, but when the user is not currently selecting a slots, it is a bit difficult to navigate when there is no content in the <td> cells.

This is particularly important for the cells where we have already scheduled sessions. In this instance, we can navigate to the cell in the table and receive the information about the session, but not the information about what time/in which room it is going to take place.