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/authPrompt.js

@@ -140,7 +140,7 @@ var AuthPrompt = GObject.registerClass({
         this.add_child(this._mainBox);
 
         this.cancelButton = new St.Button({
-            style_class: 'modal-dialog-button button cancel-button',
+            style_class: 'login-dialog-button cancel-button',
             accessible_name: _('Cancel'),
             button_mask: St.ButtonMask.ONE | St.ButtonMask.THREE,
             reactive: this._hasCancelButton,