Fix #641 by using standard intent to select group avatar.

The ACTION_GET_CONTENT used with cropping is not supported on all devices.
To make this work more reliably I removed the cropping and MediaStore.EXTRA_OUTPUT.
The image is now read via getContentResolver().openInputStream() which should work on all device including KitKat/CM11.
Merge pull request #946 from DorianScholz/groupavatar

Use standard intent to select group avatar.