Allow downloading 2FA recovery codes as a file.

Problem to solve

Many websites after the user enables 2FA on their account will be taken to a page with backup codes similar to GitLab but they will also show a button allowing the user to download their codes as a file.

This makes it more convenient to store the codes than copying them and opening a text editor to paste and save them.

Proposal

Add a button to the 2FA recovery codes page with the text "Download" which starts a download for a file gitlab-recovery-codes.txt