Add field to skip backup for an app

This skips the app from a backup when doing a full box backup and
simply reuses the previous backup.

The app can still be explicitly backed up using 'cloudron backup'
and explicitly restored using 'cloudron restore --backup'.

When restoring the box, it all depends on the app's last backup.

Fixes #311

src/routes/apps.js

@@ -58,7 +58,8 @@ function removeInternalAppFields(app) {
         xFrameOptions: app.xFrameOptions,
         sso: app.sso,
         debugMode: app.debugMode,
-        robotsTxt: app.robotsTxt
+        robotsTxt: app.robotsTxt,
+        enableBackup: app.enableBackup
     };
 }
 
@@ -130,6 +131,7 @@ function installApp(req, res, next) {
     if (data.xFrameOptions && typeof data.xFrameOptions !== 'string') return next(new HttpError(400, 'xFrameOptions must be a string'));
 
     if ('sso' in data && typeof data.sso !== 'boolean') return next(new HttpError(400, 'sso must be a boolean'));
+    if ('enableBackup' in data && typeof data.enableBackup !== 'boolean') return next(new HttpError(400, 'enableBackup must be a boolean'));
 
     if (('debugMode' in data) && typeof data.debugMode !== 'object') return next(new HttpError(400, 'debugMode must be an object'));
 
@@ -171,6 +173,8 @@ function configureApp(req, res, next) {
     if (data.altDomain && typeof data.altDomain !== 'string') return next(new HttpError(400, 'altDomain must be a string'));
     if (data.xFrameOptions && typeof data.xFrameOptions !== 'string') return next(new HttpError(400, 'xFrameOptions must be a string'));
 
+    if ('enableBackup' in data && typeof data.enableBackup !== 'boolean') return next(new HttpError(400, 'enableBackup must be a boolean'));
+
     if (('debugMode' in data) && typeof data.debugMode !== 'object') return next(new HttpError(400, 'debugMode must be an object'));
 
     if (data.robotsTxt && typeof data.robotsTxt !== 'string') return next(new HttpError(400, 'robotsTxt must be a string'));