[e.cash] Mobile nav menu improvments

Summary:
Adding the social links to the mobile nav and fixing a couple issues
-allow scrolling if content is taller than the screen
-add dropdown indicators to dropdown items

Test Plan: Preview the site and check the mobile menu. Test the social links and see if any issues

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15745