Support password managers for multiple retros

[davidje13]

Password managers attempt to auto-fill the retro password field, but store a single password regardless of the actual retro being accessed.

It looks like a hidden input element with the retro ID would help at least some password managers understand the form, but it is not clear how well supported this approach is. Also the example uses a regular email field which is hidden via CSS, but it is worth checking if this would also work with a regular type="hidden" field with an appropriate name (according to the spec it should be valid to mark a hidden field with autofill="username", and when applied in this way autofill describes the meaning of the value, not a field to be auto-populated).

ID should be used rather than slug so that moving a retro does not invalidate saved passwords.

This change should also include replacing the current autocomplete="off" on the password (which was originally intended to avoid this problem but is apparently ignored), with autocomplete="current-password". It may also be semantically useful to mark the field as required.

[davidje13]

This has been implemented now, but need to confirm that it actually works with the various password managers users are using.