Refactor integration manager handling into a common place

It was already in a common place, but this is the boilerplate for supporting multiple integration managers, and multiple integration manager sources. 

For https://github.com/vector-im/riot-web/issues/4913 / https://github.com/vector-im/riot-web/issues/10161

src/integrations/IntegrationManagers.js

@@ -0,0 +1,68 @@
+/*
+Copyright 2019 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import SdkConfig from '../SdkConfig';
+import sdk from "../index";
+import Modal from '../Modal';
+import {IntegrationManagerInstance} from "./IntegrationManagerInstance";
+
+export class IntegrationManagers {
+    static _instance;
+
+    static sharedInstance(): IntegrationManagers {
+        if (!IntegrationManagers._instance) {
+            IntegrationManagers._instance = new IntegrationManagers();
+        }
+        return IntegrationManagers._instance;
+    }
+
+    _managers: IntegrationManagerInstance[] = [];
+
+    constructor() {
+        this._setupConfiguredManager();
+    }
+
+    _setupConfiguredManager() {
+        const apiUrl = SdkConfig.get()['integrations_rest_url'];
+        const uiUrl = SdkConfig.get()['integrations_ui_url'];
+
+        if (apiUrl && uiUrl) {
+            this._managers.push(new IntegrationManagerInstance(apiUrl, uiUrl));
+        }
+    }
+
+    hasManager(): boolean {
+        return this._managers.length > 0;
+    }
+
+    getPrimaryManager(): IntegrationManagerInstance {
+        if (this.hasManager()) {
+            // TODO: TravisR - Handle custom integration managers (widgets)
+            return this._managers[0];
+        } else {
+            return null;
+        }
+    }
+
+    openNoManagerDialog(): void {
+        // TODO: Is it Integrations (plural) or Integration (singular). Singular is easier spoken.
+        const IntegrationsManager = sdk.getComponent("views.settings.IntegrationsManager");
+        Modal.createTrackedDialog(
+            "Integration Manager", "None", IntegrationsManager,
+            {configured: false}, 'mx_IntegrationsManager',
+        );
+    }
+}