This commit includes the major changes between DAVdroid and EteSync. It
adjusts the app to use the EteSync protocol and server. It includes some
ugliness still, and it's a squash of many ugly snapshot commits while
hacking on the initial DAVdroid code.
History should be "clean" from this point onwards.
* 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