Case sensitivity phase 1

I think this looks ready, and all the tests pass.

As discussed [on the list](https://groups.google.com/forum/#!msg/mozilla.dev.identity/tvQdyLbfyH8/YSwiNkW5Kj8J) back in January (wow, it's been a while), phase 1 of fixing email normalization is to be consistent about applying the existing rules. Phase 2 is deciding if we want to change the normalization scheme, and making any changes. This patch covers phase 1.

Because our handling of email case has been inconsistent, we have a number of bugs that should be fixed by this patch: #2891, #2866, #2049, #2551, #1104.

There are a number of related bugs suggesting one approach or another, or suggesting that we explicitly define our approach, and those aren't addressed yet.