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

spec/mailers/user_notifications_spec.rb

@@ -121,7 +121,7 @@ describe UserNotifications do
   end
 
   describe '.email_login' do
-    let(:email_token) { user.email_tokens.create!(email: user.email).token }
+    let(:email_token) { Fabricate(:email_token, user: user, scope: EmailToken.scopes[:email_login]).token }
     subject { UserNotifications.email_login(user, email_token: email_token) }
 
     it "generates the right email" do