correct charset for contact column

Fixes #819.

sa/_db/migrations/20151002162726_CorrectDefaultUTFCharset.sql

@@ -0,0 +1,12 @@
+
+-- +goose Up
+-- SQL in section 'Up' is executed when this migration is applied
+
+-- Adjust utf8mb4 is the real 4-byte UTF-8. But to fit the contact column in an
+-- index entirely, we need to adjust 255 down to 191.
+ALTER TABLE `registrations` MODIFY COLUMN contact varchar(191) CHARACTER SET utf8mb4 NOT NULL;
+
+-- +goose Down
+-- SQL section 'Down' is executed when this migration is rolled back
+
+ALTER TABLE `registrations` MODIFY COLUMN contact varchar(255) CHARACTER SET utf8 NOT NULL;