Code integrity check

There are currently multiple problems with the code integrity check, which should be tackled at some point:

1. Integrity is only checked after an update, but for everything
2. Integrity of an app is not checked after an update from the appstore
3. The code of the app is run (updater) before doing the integrity check.
4. The integrity check is currently synchronous while running the update (maintenance mode on) but takes a lot of time

Things that should change:

1. Integrity check should be run before the update (of server and app), so we only run signed code (although for the server this is somewhat unrealistic, because you replaced the files before already, so an attack case can not be covered, only a "borked" upload.
2. Integrity check should be ran for the specific app when updating via the app store
3. When updating via console the integrity check should be optional, to allow faster deployment across multiple instances when you use a deployment tool (chef, puppet, git, ...).

cc @MorrisJobke @rullzer

Start by tracing the server and app update entry points, including App Store and console update paths, and locate the existing integrity check. Compare when checks run in each path and how maintenance mode affects them. Done means checks cover the requested update flows, console checks are optional, and the timing avoids executing updated code before validation.