Attribute support replies to their sender via message metadata

[jeremy46231]

Adds .metadata.event_payload.source_user_id wherever the support relay posts with a custom name and avatar.

       username: display_name,
-      icon_url: avatar_url
+      icon_url: avatar_url,
+      metadata: {
+        event_type: "forge_message",
+        event_payload: { source_user_id: slack_user_id }
+      }
     )

Same in admin/support_tickets_controller.rb. slack-ruby-client JSON-encodes metadata itself, same as blocks.

For ShowRealUser in Taut (it renders bot-sent messages as the user that actually sent them), and it's also useful for this metadata to exist for other uses. The same thing is already in at-channel, prometheus, slack-extra and others.