Add a maintenance mode for running application

[ckaipf]

This PR wants to add

• a tween which temporarily redirects all requests to 503
• state is implemented by the presence of local file
• if file is present maintenance mode is off, if file is absent maintenance mode is on
  • an argument for this implementation was that it can provide a sanity check for the underlying file system, e.g. file lost due to non mounted file system
• also added excludable routes, idea behind this is the implementation of administrative endpoints to toggle mode