Bug903918 - Implement two column layout for tablet

Implement two column layout for tablet.
Split into two commit:
5aeb5b6 Add shared styles of building block in two_column.css
7a6ef4f [Flatfish][Settings] support 2 column layout for tablet

the first commit 5aeb5b6 could be removed once the patch to [Bug 905065](https://bugzilla.mozilla.org/show_bug.cgi?id=905065) has landed.


[Link to Bug 903918 - [Flatfish][Settings] support 2 column layout for tablet](https://bugzilla.mozilla.org/show_bug.cgi?id=903918)