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

Fixes: #7876 (closed)