* remove Android 4 support
* update WebRTC
* show channel avatars and number of users for channel discovery
* add ability to ex-/import app settings
* implementation of message retraction XEP-0424
* implement message history for corrected messages
* add text formatting buttons in chat
* hide text formatting in chats
* add ability to delete own avatar
* show black bars when remote video does not match aspect ratio of screen
* improve search performance
* bug fixes
* removed OTR again
