password verification after clicking on the Show secret variables button

Ask for another level of verification to check the user's identity. Much like what google and other services do.

This issue came forth from https://gitlab.com/gitlab-org/gitlab-ce/issues/21358#note_19121066

proposal

this feature would be a mix between:

hide-vars gif
and Secret_Variable_hidden Secret_Variable_unlock

cc: @markpundsack @ayufan