Recent commit [1] added a strong light border around user avatar
icons, in accordance with design mockups.
As a probably unintentional side-effect, the border was also added
around the symbolic fallback icon, which is displayed whenever the
user avatar is not available. This doesn't work well with the current
design, as the strong border makes the subtle fallback icon
background indistinguishable. Additionally, it doesn't match the
design mockups for the symbolic avatar icon [2].
Correct this by adding a style class for when avatar image is used,
and apply the border only for that case.
[1] 498710c2ec
[2] https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/lock-login/username-based-login.png
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1490>
a7c343292e
