* use cert4android instead of MemorizingTrustManager
* new app setting: distrust system certificates
* add network security config to manifest so that user-installed CAs will be accepted in Android 7 again
* update gradle
* re-initialize logger in :sync process after changing the settings (IPC using broadcast)
* move settings from SharedPreferences (which is not multi-process-safe) to ServiceDB
* logger: show exception details
* settings: show debug info
* add startup dialogs (F-Droid: donations, Play Store: DRM bug, OpenTasks not installed)
* allow to reset hints/startup dialogs
* AccountSettings: fragment as inner class of activity