Subtask of #1245 (closed).

These specifications are a summary of the discussion in #1247 (closed). Please look there for details:

• Give the user the possibility to lock the app
• This is different from signing out, in the sense that the user will stay online and can receive messages
• Only the content of the app will be inaccessible
• This PIN lock feature should be optional and needs to be activated by the user
• Ideas for how to lock the app:
  • action in navigation drawer
  • action button below the permanent "signed in" notification
  • panic response (out of scope of this ticket) #1345
• There should also be an option to automatically lock the app after a period of time. Maybe the user can define this time.
• It should be easy for the user to unlock the app. Ideally they don't need to remember another PIN or password. Signal uses the existing lockscreen security mechanism (secret or fingerprint) for unlocking. We can do the same. If there is no lockscreen security set, we can deactivate the feature.
• Open question: How to handle notifications when app is locked?
  • option 1: show them as they are now, but ask for unlocking when they are clicked (Signal is doing this)
  • option 2: don't show any notifications while the app is locked