Enhanced auth token / remember me (#27606)

Closes #27455

> The mechanism responsible for long-term authentication (the 'remember
me' cookie) uses a weak construction technique. It will hash the user's
hashed password and the rands value; it will then call the secure cookie
code, which will encrypt the user's name with the computed hash. If one
were able to dump the database, they could extract those two values to
rebuild that cookie and impersonate a user. That vulnerability exists
from the date the dump was obtained until a user changed their password.
> 
> To fix this security issue, the cookie could be created and verified
using a different technique such as the one explained at
https://paragonie.com/blog/2015/04/secure-authentication-php-with-long-term-persistence#secure-remember-me-cookies.

The PR removes the now obsolete setting `COOKIE_USERNAME`.

docs/content/administration/config-cheat-sheet.en-us.md

@@ -517,7 +517,6 @@ And the following unique queues:
 - `SECRET_KEY`: **\<random at every install\>**: Global secret key. This key is VERY IMPORTANT, if you lost it, the data encrypted by it (like 2FA secret) can't be decrypted anymore.
 - `SECRET_KEY_URI`: **_empty_**: Instead of defining SECRET_KEY, this option can be used to use the key stored in a file (example value: `file:/etc/gitea/secret_key`). It shouldn't be lost like SECRET_KEY.
 - `LOGIN_REMEMBER_DAYS`: **7**: Cookie lifetime, in days.
-- `COOKIE_USERNAME`: **gitea\_awesome**: Name of the cookie used to store the current username.
 - `COOKIE_REMEMBER_NAME`: **gitea\_incredible**: Name of cookie used to store authentication
    information.
 - `REVERSE_PROXY_AUTHENTICATION_USER`: **X-WEBAUTH-USER**: Header name for reverse proxy