Fix #162 Record the position in the list of each voted session

[adrianbanks]

This will require a schema change:

ALTER TABLE [dbo].[Votes] ADD [PositionInList] [int] NOT NULL

As a result, it will need some level of integration with #266, as that contains a script to create the default structure.

[alastairs]

266 is now merged.

[adrianbanks]

I've rebased this onto the database migrations branch, so it should merge more easily.

[alastairs]

Let's see how well the migrations work...