Redirect user to sign_in page after sign_out
Redirect user to sign_in page after sign_out

Before this change user was redirected to index page which requires
authentication. This caused a 'Signed out successfully' message to be
overriden with 'You need to sign in' message. It was quite confusing
for a user to see 'You need to sign in' message right after signing
out.

alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2