Signed-in users with expired password can reset it

www.agileplannerapp.com/boards/173808/cards/8676
fixes: https://govuk.zendesk.com/agent/tickets/874886

by default `require_no_authentication` filter will redirect a
partially signed-in user to the after sign-in path, i.e. '/'
where expirable will intercept and ask the user to change
their password if it has expired.

in case of the zen ticket mentioned above, a user with an
expired password is unable to reset it, because clicking on
the reset password link takes them to the change password
screen asking them to input their current password (which
they don't remember).

we're signing-out the user in this particular situation
when they click on the reset password link, so that they're
straight-away taken to the reset password screen.
Link to passwords#new from change password view

www.agileplannerapp.com/boards/173808/cards/8676

following-up on the previous commit 17a185f, this change
adds link on the change password screen to sign-out the
current user so that they can request reset passphrase
instructions.
Integration tests to cover user flows

www.agileplannerapp.com/boards/173808/cards/8707
Merge pull request #294 from alphagov/allow-users-with-expired-password-to-reset

Signed-in users with expired password can reset it