Implement MSC1957: Integration manager discovery

https://github.com/matrix-org/matrix-doc/pull/1957

Yay Riot iOS now supports integration manager discovery!

roles/matrix-base/templates/static-files/well-known/matrix-client.j2

@@ -8,4 +8,14 @@
 		"base_url": "{{ matrix_identity_server_url }}"
 	}
 	{% endif %}
+	{% if matrix_dimension_integrations_rest_url and matrix_dimension_integrations_ui_url %},
+	"m.integrations": {
+		"managers": [
+			{
+				"api_url": "{{ matrix_dimension_integrations_rest_url }}",
+				"ui_url": "{{ matrix_dimension_integrations_ui_url }}"
+			}
+		]
+	}
+	{% endif %}
 }