Feature/502 Update the header and main navigation

[dlicheva]

Ticket This PR updates the style and content of the main navigation.

Changes:

• add a new main nav link: "Browse"
• style the header and show it on the home page too. Remove the search from the header only on the home page, to avoid duplication.
• add "OR" to the homepage to separate Browse from Search
• adjust page titles (Root CREs and Map Analysis)
• tweak the margins and padding on the pages, so they align with the logo nicely
• Style the header for mobile

Screenshots

Map Analysis page

Browse page

Home page