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.

I had considered that, but then it's a bit weird as to why the schema says it is 5 with no way to document the reason. It might be better to have the default set when a new conference is created, then it can be documented in code.