[explorer] Mobile navbar improvements

Summary:
The navbar was getting cramped on smaller screens. The search bar can go off the right side and the logo and network select can get bunched up.
Proposed edits to move the mempool select and info link into a mobile nav menu, and fix some issues with the search box overflowing

Test Plan: preview and check

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D19206