Documentation incorrectly suggests U2F is only supported by Chrome

Various parts of GitLab claim that Google Chrome is the only browser supporting U2F as 2FA method. This has been incorrect for some time now: FireFox has supported U2F since v57 (https://www.reddit.com/r/firefox/comments/7dbcbl/firefox_quantum_comes_with_u2f_support/).

References are here:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/profile/account/two_factor_authentication.md#L21
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/u2f/_register.html.haml#L5
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/locale/gitlab.pot#L7773