emails: add fix script to delete duplicate email addresses.

In the past it was possible for users to create multiple accounts with
the same email address. We had about 9000 such accounts. This removes
the email address from these accounts.

When multiple accounts have the same email address, the account that
visited the site last gets to keep the address.

app/models/user.rb

@@ -149,6 +149,7 @@ class User < ApplicationRecord
   scope :deleted, -> { where("name ~ 'user_[0-9]+~*'") }
   scope :undeleted, -> { where("name !~ 'user_[0-9]+~*'") }
   scope :admins, -> { where(level: Levels::ADMIN) }
+  scope :banned, -> { bit_prefs_match(:is_banned, true) }
 
   scope :has_blacklisted_tag, ->(name) { where_regex(:blacklisted_tags, "(^| )[~-]?#{Regexp.escape(name)}( |$)", flags: "ni") }
   scope :has_private_favorites, -> { bit_prefs_match(:enable_private_favorites, true) }