Improve mobile experience

[hslee2008]

Problem

Rust playground will be much more better if it's a mobile friendly editor. Though it is true mobiles are not for editors, people viewing documentations in mobile might be uncormftable.

The UI were very small or sometimes it went off-screen.

How to fix

1. Update index.ejs (add viewport for mobile)
2. Update the header ui to make a 'more' button in mobiles that loads other contents if clicked