[#793] Drifting: mobile style

* only split into two-columns on larger screens

* tweak borders so that they work on both one-column and two-column views

* force #primary (entries) to come above #secondary (the sidebar) in one-column view

* print out the meta viewport tag

Fixes #793
Merge pull request #812 from afuna/drifting-mobile-bug-793

[#793] Drifting: mobile style