Fix #199, fix migrations

Deleted migration that was renaming and dropping user columns, the columns were changed in an earlier migration or created with the correct name.

Adjusted the changing of the datatype of the "conferences:day_start" and "conferences:day_end" columns. PostgreSQL did not know how to convert a time into an integer, and because of that refused to change the type. To be database independent, I did not use the PostgreSQL way by explaining how to convert with USING CAST(EXTRACT(hour from :day_start) AS integer). I chose to create a new column, drop the old one and rename the new column to the old name.