Fix missing API functions being used on the background page

ext/bg/background.html

@@ -20,9 +20,9 @@
 
         <script src="/mixed/js/core.js"></script>
         <script src="/mixed/js/dom.js"></script>
-        <script src="/mixed/js/api.js"></script>
 
         <script src="/bg/js/anki.js"></script>
+        <script src="/bg/js/api.js"></script>
         <script src="/bg/js/mecab.js"></script>
         <script src="/bg/js/audio.js"></script>
         <script src="/bg/js/backend-api-forwarder.js"></script>

ext/bg/js/api.js

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2019  Alex Yatskov <alex@foosoft.net>
+ * Author: Alex Yatskov <alex@foosoft.net>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+
+function apiTemplateRender(template, data, dynamic) {
+    return _apiInvoke('templateRender', {data, template, dynamic});
+}
+
+function apiAudioGetUrl(definition, source, optionsContext) {
+    return _apiInvoke('audioGetUrl', {definition, source, optionsContext});
+}
+
+function _apiInvoke(action, params={}) {
+    const data = {action, params};
+    return new Promise((resolve, reject) => {
+        try {
+            const callback = (response) => {
+                if (response !== null && typeof response === 'object') {
+                    if (typeof response.error !== 'undefined') {
+                        reject(jsonToError(response.error));
+                    } else {
+                        resolve(response.result);
+                    }
+                } else {
+                    const message = response === null ? 'Unexpected null response' : `Unexpected response of type ${typeof response}`;
+                    reject(new Error(`${message} (${JSON.stringify(data)})`));
+                }
+            };
+            const backend = window.yomichan_backend;
+            backend.onMessage({action, params}, null, callback);
+        } catch (e) {
+            reject(e);
+            yomichan.triggerOrphaned(e);
+        }
+    });
+}