ConfigAPI: new class to handle frontend requests to config backend

This new API is somewhat patterned after the InstallAPI, but simpler
whenever possible.

app/common/ConfigAPI.ts

@@ -0,0 +1,31 @@
+import {BaseAPI, IOptions} from "app/common/BaseAPI";
+import {addCurrentOrgToPath} from 'app/common/urlUtils';
+
+/**
+ * An API for accessing the internal Grist configuration, stored in
+ * config.json.
+ */
+export class ConfigAPI extends BaseAPI {
+  constructor(private _homeUrl: string, options: IOptions = {}) {
+    super(options);
+  }
+
+  public async getValue(key: string): Promise<any> {
+    return (await this.requestJson(`${this._url}/api/config/${key}`, {method: 'GET'})).value;
+  }
+
+  public async setValue(value: any, restart=false): Promise<void> {
+    await this.request(`${this._url}/api/config`, {
+      method: 'PATCH',
+      body: JSON.stringify({config: value, restart}),
+    });
+  }
+
+  public async restartServer(): Promise<void> {
+    await this.request(`${this._url}/api/admin/restart`, {method: 'POST'});
+  }
+
+  private get _url(): string {
+    return addCurrentOrgToPath(this._homeUrl);
+  }
+}