HOLD: Move New button into navbar as Create dropdown

[maebeale]

What is the goal of this PR and why is this important?

• The standalone "New" button felt disconnected from the main navigation
• Moving it into the nav bar as a "Create" dropdown makes it consistent with the other menu items (Curriculum, Community, Help)

How did you approach the change?

• Moved the creation links from _navbar_new_button into _navbar_menu as a new "Create" dropdown, styled identically to the existing nav dropdowns
• Positioned "Create" before "Help" since it's a primary action for signed-in users
• Updated the mobile menu to match: renamed "New" to "Create" and reordered so Create appears before Help
• Removed the separate _navbar_new_button render from _navbar.html.erb

UI Testing Checklist

• Desktop: "Create" dropdown appears between "Community" and "Help" when signed in
• Desktop: "Create" dropdown is hidden when not signed in
• Desktop: All creation links work correctly from the dropdown
• Desktop: Admin-only items still show with blue background for admin users
• Mobile: "Create" section appears between "Community" and "Help" when signed in
• Mobile: "Create" section is hidden when not signed in

Anything else to add?

• The _navbar_new_button.html.erb partial is now unused and can be deleted in a follow-up

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