The edit dialog of a secret was filled with its current value, so every
readable secret's value was in the secrets page's HTML. A private
secret, or one with no value yet, cannot be read: its field held the
text "None", which is valid base64, and saving the dialog unchanged set
the secret to the three bytes it decodes to. The dialog now only writes
a new value: the field starts empty, is a password field and is
required. The page no longer reads secret values, and get_secret_value,
used only for that, is gone.
