Add readonlyRootfs flag to apps table

When turned off, it will put the app in a writable rootfs. This
allows us to debug live/production apps (like change start.sh) and
just get them up and running. Once turned off, this app cannot be
updated anymore (unless the force flag is set). This way we can
then update it using the CLI if we are convinced that the upcoming
update fixes the problem.

Part of #171

migrations/schema.sql

@@ -67,6 +67,7 @@ CREATE TABLE IF NOT EXISTS apps(
     altDomain VARCHAR(256),
     xFrameOptions VARCHAR(512),
     sso BOOLEAN DEFAULT 1, // whether user chose to enable SSO
+    readonlyRootfs BOOLEAN DEFAULT 1, // readonly rootfs
 
     lastBackupId VARCHAR(128), // tracks last valid backup, can be removed