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.
* honour calendar-proxy-read/write-for property
* ignore errors when quering member groups for home sets
* remove home sets and collections from the service database not only on 404, but 403, 404 and 410
* fix crash bug when <displayname> was defined, but empty