Implement changes to nav bar

Created by: felixfbecker

Specification: Navigation section in Information Architecture document Product issue: #11775 Design: Figma Design owner: @AlicjaSuska

• Move extensions item from user menu to navbar, add icon
• Always show an avatar, never the username. If we don't have an avatar image URL, show a circle avatar with the initials.
• Change icon of command palette
• Remove explore page (completely)
• Add 'Sign up' primary button for un-authenticated users
• Change 'Sign in' button label to 'Log in'. Change button style to secondary.