fix migration syntax and separate select and update

Migration is still crazy slow on we dev. Might return to non sql ruby
join MessagePages and UserParticipations instead of sub select

The subselect was taking very long because it would consider every page not just the MessagePages.
Merge pull request #73 from Crabgrass/bugfix/fix-migration-20100603010101

Bugfix/fix migration 20100603010101