The `oauth_state` cookie used to be created with the SameSite value set
according to the `cookie_samesite` configuration.
However, due to a Safari bug SameSite=None or SameSite=invalid are treated
as Strict which results in "missing saved state" OAuth login failures
because the cookie is not sent with the redirect requests to the OAuth
provider.
This commit always creates the `oauth_state` cookie with SameSite=Lax
to compensate for this.