escape paths for use with commandline

escape qualityPath and fullOutput for use with commandline
In order to successfully download files from subscriptions these strings should be  escaped to work properly in the commandline. 
I have seen you use almost the same function (generateArgs()) in app.js. Even though I have never had a problem with this outside subscriptions I would suggest to do the same for that function starting on line 1405

backend/subscriptions.js

@@ -366,11 +366,11 @@ async function generateArgsForSubscription(sub, user_uid, redownload = false, de
 
     let appendedBasePath = getAppendedBasePath(sub, basePath);
 
-    let fullOutput = `${appendedBasePath}/%(title)s.%(ext)s`;
+    let fullOutput = `'${appendedBasePath}/%(title)s.%(ext)s'`;
     if (desired_path) {
-        fullOutput = `${desired_path}.%(ext)s`;
+        fullOutput = `'${desired_path}.%(ext)s'`;
     } else if (sub.custom_output) {
-        fullOutput = `${appendedBasePath}/${sub.custom_output}.%(ext)s`;
+        fullOutput = `'${appendedBasePath}/${sub.custom_output}.%(ext)s'`;
     }
 
     let downloadConfig = ['-o', fullOutput, !redownload ? '-ciw' : '-ci', '--write-info-json', '--print-json'];
@@ -381,8 +381,8 @@ async function generateArgsForSubscription(sub, user_uid, redownload = false, de
         qualityPath.push('-x');
         qualityPath.push('--audio-format', 'mp3');
     } else {
-        if (!sub.maxQuality || sub.maxQuality === 'best') qualityPath = ['-f', 'bestvideo[ext=mp4]+bestaudio[ext=m4a]/mp4'];
+        if (!sub.maxQuality || sub.maxQuality === 'best') qualityPath = ['-f', '\'bestvideo[ext=mp4]+bestaudio[ext=m4a]/mp4\''];
-        else qualityPath = ['-f', `bestvideo[height<=${sub.maxQuality}]+bestaudio/best[height<=${sub.maxQuality}]`, '--merge-output-format', 'mp4'];
+        else qualityPath = ['-f', `'bestvideo[height<=${sub.maxQuality}]+bestaudio/best[height<=${sub.maxQuality}]'`, '--merge-output-format', 'mp4'];
     }
 
     downloadConfig.push(...qualityPath)