Update navigation menu and footer

[traceyljohnson]

We've redesigned the navigation menu!

Please implement following the Figma specs here.

• I've included specs for all hover states and interactions.
• Everything should inherit the latest typography, spacing, and button styles.
• Let me know if anything is unclear!

For the footer, please reference this mockup

[gitstart-app[bot]]

Here is the GitStart Ticket for this issue: https://clients.gitstart.com/sourcegraph/3436/tickets/SGM-7012

[traceyljohnson]

@gitstart-sourcegraph We also noticed an issue with the text color in the footer. Please make sure it gets resolved in this PR :)