There are a few places in the code that use the UserAvatar widget
without using the user menu.
Since the avatar is used outside of the user menu, move it from
userMenu.js to its own file.
The user widget is the username and avatar shown on
the unlock dialog.
The login dialog has something very similar.
This commit separates the user widget out to its own
file, so we can use it from the login dialog in a
later commit.
https://bugzilla.gnome.org/show_bug.cgi?id=694062