Revert case sensitivity for email uniqueness (#930)

In #889, `case_sensitive` setting of the `uniqueness` validator was set
to `false` to prevent deprecation warnings from Rails.

However, the setting should have been set to `true` to preserve
backwards-compatibility.

The deprecation warning said:

> To **continue** case sensitive comparison
> on the :email attribute in User model, pass case_sensitive: true
> option explicitly to the uniqueness validator.

Setting it to `false`, made ActiveRecord generate User queries on
`LOWER(email)` instead of on `email`. The apps that didn't have an index
on `LOWER(email)` would have faced performance issues.

This was mentioned in an old commit that originally made the switch from
`false` to the implicit `true`:
56f0f04

lib/clearance/user.rb

@@ -152,7 +152,7 @@ module Validations
         validates :email,
           email: { strict_mode: true },
           presence: true,
-          uniqueness: { allow_blank: true, case_sensitive: false },
+          uniqueness: { allow_blank: true, case_sensitive: true },
           unless: :email_optional?
 
         validates :password, presence: true, unless: :skip_password_validation?