Ref T4443. In addition to performing upgrades from, e.g., md5 -> bcrypt, also allow sidegrades from, e.g., bcrypt(cost=11) to bcrypt(cost=12). This allows us to, for example, bump the cost function every 18 months and stay on par with Moore's law, on average.

I'm also allowing "upgrades" which technically reduce cost, but this seems like the right thing to do (i.e., generally migrate password storage so it's all uniform, on average).

• Fiddled the bcrypt cost function and saw appropriate upgrade UI, and upgraded passwords upon password change.
• Passwords still worked.
• Around cost=13 or 14 things start getting noticibly slow, so bcrypt does actually work. Such wow.