Add Preferences entry to the bottom of Vertical Navigation

[mklanjsek]

Navigation redesign (#1475) amplified a need for UI to handle the client side preferences. To start with, here is initial set of preferences users will be able to modify:

• Light/Dark theme,
• Collapsed/Expanded navigation state,
• Show labels in left-most navigation

Preferences popup will be accessed by clicking the Preferences button at the bottom of the left-most navigation panel. In same way as Theme, all other prefs should persist between sessions in browser storage.