Require the user to call `authenticateInstallation` manually

This solves multiple issues:
- Possible crashes at startup because the app is already in the process of presenting another modal view. This way the developer can make sure that only one modal view is being presented
- If the app is showing e.g. a login view on startup, it is now not needed to turn off automatic mode and setup the complete auth workflow manually and simply invoke `authenticateInstallation` after the login view is either fully presented (`viewDidLoad` finished) or the user did log in
Remove `automaticMode` property since it is not needed any more

The developer needs to call at least `authenticateInstallation` instead.
Add step to setup BITAuthenticator in installation guides
Merge branch 'feature/authSetup' into develop
re-validate once alert-view was dismissed

* factor out common validate method
* call it in authenticate and after validation-error was dismissed
* don't reset identification if validation failed - it might have only been
a short network outage.
add missing localized error message

if there's no network response, also show a proper error.