Restyle buttons (default size)

Changed appearance of btn-default, btn-primary and btn-secondary.
Added some btw variables to assist with styling.
Removed disabled pseudo styling for btn-default for now.
Restyle btn-large

I couldn’t get the math operation to subtract 2px from the variables that
are needed to size .btn-default correctly, so I hard-coded the numbers - 2px…
Merge pull request #191 from ESWAT/button-restyle

Style buttons to match new design