mu should not be unsigned

[Janzert]

In website/sql/schema.sql User table, mu is defined as an unsigned float. Trueskill mu though can go negative. Looking at the current rankings it appears there are a number of users that have hit this limit.

This is probably part of the reason that the median mu for the competition seems to be slowly creeping up over time.

[truell20]

Thanks for the report. Will fix.