ParseSlackTokenResponse treated the incoming_webhook field as
mandatory, which blocked any Slack OAuth2 flow that did not request
the incoming-webhook scope. Access review Slack connects only ask
for users:read and users:read.email and would fail at token parsing.
Treat incoming_webhook as optional: populate SlackSettings when it
is present, leave them empty otherwise. The existing compliance-page
webhook URL is preserved through Reconnect in a later commit.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
62bab0f732