TOTP programs generally group the code into two groups of 3 digits,
but we'd only allow a single group of 6 digits.
Allow spaces in the value for a bit of flexibility, and fix
placeholders to look like codes (also turns out @placeholder on a
field doesn't do anything, not sure where I got this idea).
Also improve the label slightly in the login flow:
* add information to the label itself
* properly link the label & input via an `id`
Xavier Morel
70b6ac5009