Bug903918 - Implement two column layout for tablet r=crh0716

Implement two column layout for tablet.
Split into two commit:
201756e4b4cdf58f048a979d19a010575292c450 Add shared styles of building block in settings_large.css
8f1fe2d859c17450764c04152e236589c1868c7a [Flatfish][Settings] support 2 column layout for tablet

the commit 201756e4b4cdf58f048a979d19a010575292c450 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)