Updated to fix button line-height across all buttons sizes.

As there are some issues with other buttons sizes line-heights which I missed. This should be the ultimate fix for all sizes.

1. As the base size for calculating button sizes is `6px` I decided to change the calculation to use this. Now it is just `$base-size * 3 4 5 6` which is a little bit easier to work with.

2. The biggest issue is causing `13px` size of the Guardian Web Sans font so this sizes need line-height adjustment for less/more than desktop.

This is so far the nest solution I could come up with. It is possible that I will change resolution from which we will download this font to user device. Now it's set to desktop (980px) and it can possibly be moved down to 780px so it will actually download the font on tablets but still not on most of the phones. But I need to properly check stats first.

@ironsidevsquincy @austinh7 @mattosborn