Use the database for the new URL whitelist

This means that we can validate user input in the transition app without
duplicating the whitelist (or having some other, more elaborate sharing
mechanism).

It will mean an extra query for every redirect Bouncer serves...
Merge pull request #69 from alphagov/move_whitelist_to_db

Use the database for the new URL whitelist
Use the new Site.global_type column

...instead of global_http_status. This also includes Transition's
current db/structure.sql, which has some bits of accidentally-committed
collation diff, but since that should be sorted out in transition first
I've copied it all over as-is.