Add username to unknown sign in mail

What does this MR do and why?

This MR adds both full user name and username to unknown sign in HTML and text emails.

Related issues

• Related to Show username in sign-in notification emails (#225183 - closed)

Screenshots or screen recordings

before	after
Hi Simonne Runolfsson! A sign-in to your account has been made from the following IP address: 127.0.0.1 If you recently signed in and recognize the IP address, you may disregard this email. If you did not recently sign in, you should immediately change your password: http://127.0.0.1:3000/help/user/profile/user_passwords#change-your-password. Passwords should be unique and not used for any other sites or services. To further protect your account, consider configuring a two-factor authentication method: http://127.0.0.1:3000/help/user/profile/account/two_factor_authentication. -- You're receiving this email because of your account on 127.0.0.1.	Hi Simonne Runolfsson (reported_user_22)! A sign-in to your account has been made from the following IP address: 127.0.0.1 If you recently signed in and recognize the IP address, you may disregard this email. If you did not recently sign in, you should immediately change your password: http://127.0.0.1:3000/help/user/profile/user_passwords#change-your-password. Passwords should be unique and not used for any other sites or services. To further protect your account, consider configuring a two-factor authentication method: http://127.0.0.1:3000/help/user/profile/account/two_factor_authentication. -- You're receiving this email because of your account on 127.0.0.1.

How to set up and validate locally

Apply these changes and visit http://localhost:3000/rails/mailers/notify/unknown_sign_in_email.html.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.