feat(mx-puppet-slack): add support for OAuth client ID/secret

The OAuth credentials method seems to be the only viable way to
configure the mx-puppet-bridge now. Legacy tokens can no longer be
created, and the other methods (xoxs and xoxc tokens) come with warnings
about them being against Slack's terms of service.

roles/matrix-bridge-mx-puppet-slack/templates/config.yaml.j2

@@ -18,6 +18,10 @@ bridge:
 # Slack OAuth settings. Create a slack app at https://api.slack.com/apps
 oauth:
   enabled: true
+  # Slack app credentials.
+  # N.B. This must be quoted so YAML does not parse it as a float.
+  clientId: '{{ matrix_mx_puppet_slack_oauth_client_id }}'
+  clientSecret: '{{ matrix_mx_puppet_slack_oauth_client_secret }}'
   # Path where to listen for OAuth redirect callbacks.
   redirectPath: {{ matrix_mx_puppet_slack_redirect_path }}
   # Set up proxying from https://your.domain/redirect_path to http://bindAddress:port/redirect_path,