Fix #289 Use the correct number of sessions for the conference

This change adds the total number of time slots and the total number of tracks to the database, and uses those values in the voting pages to visualise the data with the correct number of total sessions.

After this is merged, the number of time slots and tracks will both be set to 5 in the database.

Fair enough. Maybe set the "Number of Time Slots" default value to 5? We don't vary this as much as the number of tracks. I think that makes sense in our domain, too. What do you think?