Fix reviving revoked sessions and invalidating login (#16943)

Up until now, we have used Devise's Rememberable mechanism to re-log users
after the end of their browser sessions. This mechanism relies on a signed
cookie containing a token. That token was stored on the user's record,
meaning it was shared across all logged in browsers, meaning truly revoking
a browser's ability to auto-log-in involves revoking the token itself, and
revoking access from *all* logged-in browsers.

We had a session mechanism that dynamically checks whether a user's session
has been disabled, and would log out the user if so. However, this would only
clear a session being actively used, and a new one could be respawned with
the `remember_user_token` cookie.

In practice, this caused two issues:
- sessions could be revived after being closed from /auth/edit (security issue)
- auto-log-in would be disabled for *all* browsers after logging out from one
  of them

This PR removes the `remember_token` mechanism and treats the `_session_id`
cookie/token as a browser-specific `remember_token`, fixing both issues.

app/models/user.rb

@@ -64,7 +64,7 @@ class User < ApplicationRecord
   devise :two_factor_backupable,
          otp_number_of_backup_codes: 10
 
-  devise :registerable, :recoverable, :rememberable, :validatable,
+  devise :registerable, :recoverable, :validatable,
          :confirmable
 
   include Omniauthable