Fixes #449

BUGFIX
* Fixes a bug that happens on switching between Light and Dark mode, which results in an unreadable system status bar

ADDITIONS & CHANGES
* Adds a new `ThemeKey` enum value `system`
* Changes setting for Light/Dark mode in user profile by removing the switch
* Adds a new screen for switching application appearance to `Light`/`Dark` or `System`
* Refactors all `ThemeKey` enum values from UPPERCASE to lowercase
* Adds `vibrateLight` method for haptic feedback when user changes appearance
* Adds new assets for all three theme states: Light, Dark, System
* Cleans up code

ADDITIONAL NOTES
* Adds `activeColor` setting to all used `Switch.adaptive` widgets with the value of `CustomTheme.of(context).accent`, because on iOS they had the default green color which doesn't fit our application color theme.
* Adds `Preference` class with just two static methods `set({@required value, @required String forKey})` and `value({@required String forKey})` for convenience and better readability
* Adds static getter/setter on `Preference` class for easier handling of preference values and better readability (NOTE: Should be discussed if we adopt this system overall)
  So instead of calling:
  ```
   setPreference(preferenceAppThemeKey, ThemeKey.light.toString());
  ```
  we can call:
  ```
  Preference.themeKey = ThemeKey.light.toString();
  ```

# Conflicts:
#	lib/main.dart

* Adds missing line break after last line of code
* Fixes bug on using describeEnum(..)
* Updates assets to new versions
* Adapts corner radius at image border according Cornelius' request
* adapts UI according to Cornelius' request
* fixes Image height
* removes code regarding convenience class 'Preference'