lockscreen: Style cleanup for lock and login

- increase the font size of clock elements
- increase the size of user avatars
- combines lock and login scss into one file
- clean up the css for avatars
- adjust the blur parameters of the screen shield

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2564>

js/gdm/loginDialog.js

@@ -310,7 +310,7 @@ var SessionMenuButton = GObject.registerClass({
 }, class SessionMenuButton extends St.Bin {
     _init() {
         let button = new St.Button({
-            style_class: 'modal-dialog-button button login-dialog-session-list-button',
+            style_class: 'login-dialog-button login-dialog-session-list-button',
             icon_name: 'emblem-system-symbolic',
             reactive: true,
             track_hover: true,