style: Clean up dialogs stylesheet to fix layout issues

- remove unnecessary dialog type-specific padding, spacing and margin definitions that led to layout to problems
- bump the size of user avatar in authentication dialog
- nest css classes in the sass to better reflect the structure of dialog elements
- sync style of audio device selector with switcher-popup style
- sync text and padding styles with other elements
- fix incorrect button order in audio selection dialog

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7876
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3473>

js/ui/main.js

@@ -988,7 +988,7 @@ class RestartMessage extends ModalDialog.ModalDialog {
     _init(message) {
         super._init({
             shellReactive: true,
-            styleClass: 'restart-message headline',
+            styleClass: 'restart-message',
             shouldFadeIn: false,
             destroyOnClose: true,
         });