Generated profile pictures

Closes #163 (closed)

• Added generated SVG Avatar as profile picture
• made class ProfilePicture stateless
• added jdenticon_dart and flutter_svg (pinned to version 0.17.4 for sdk compatibility reasons) dependencies
• added field userName to ExpProfilePicture ExpProfilePicture now accepts an avatar as a string.
• added semantics for profile picture widget
• unique user name, as provided by backend, is now used for avatar generation