Updating user avatar icon to overhauled standard WikimediaUI icon.
Also:
- Using new icon default size instead of `80 x 80 px`. Icon is
resized by `background-size` in both applications anyways.
- Renaming 'user*' to 'user-avatar' for consistency across projects.
Bug: T123810
Change-Id: Ifb2027694af8f619663f00f4c50891772ee64746
