DEV: Hash tokens stored from email_tokens (#14493)

This commit adds token_hash and scopes columns to email_tokens table.
token_hash is a replacement for the token column to avoid storing email
tokens in plaintext as it can pose a security risk. The new scope column
ensures that email tokens cannot be used to perform a different action
than the one intended.

To sum up, this commit:

* Adds token_hash and scope to email_tokens

* Reuses code that schedules critical_user_email

* Refactors EmailToken.confirm and EmailToken.atomic_confirm methods

* Periodically cleans old, unconfirmed or expired email tokens

app/controllers/admin/users_controller.rb

@@ -328,7 +328,7 @@ class Admin::UsersController < Admin::AdminController
   def activate
     guardian.ensure_can_activate!(@user)
     # ensure there is an active email token
-    @user.email_tokens.create(email: @user.email) unless @user.email_tokens.active.exists?
+    @user.email_tokens.create!(email: @user.email, scope: EmailToken.scopes[:signup]) if !@user.email_tokens.active.exists?
     @user.activate
     StaffActionLogger.new(current_user).log_user_activate(@user, I18n.t('user.activated_by_staff'))
     render json: success_json